Biometric skin contact sensor and methods of operating a biometric skin contact sensor

ABSTRACT

A capacitive biometric skin contact sensor configured to resolve the contours of skin in contact with the sensor, wherein the sensor comprises: an array of sensor pixels, wherein each sensor pixel comprises a thin film transistor and a capacitive sensing electrode connected to the thin film transistor; a plurality of gate drive channels, wherein each gate drive channel is arranged to provide a gate drive signal to one or more of the sensor pixels; a plurality of read-out channels, wherein each read-out channel is arranged to receive a read-out current from one or more of the sensor pixels, each read-out current being indicative of a proximity to a respective capacitive sensing electrode of a conductive object to be sensed; and an analog to digital converter comprising a dual slope integrator arranged to receive, as its input, either an output current or a reference voltage, wherein said output current is based on one or more read-out currents; wherein the biometric sensor is configured to: apply the output current as the input to the dual slope integrator for a charging time period to charge a capacitor of the dual slope integrator; apply the reference voltage as the input to the dual slope integrator for a discharging time period, wherein the discharging time period comprises the amount of time it takes for the capacitor to discharge; and determine the proximity to the one or more respective capacitive sensing electrodes of the conductive body based on an indication of the discharging time period.

TECHNICAL FIELD

The present disclosure relates to the field of biometric skin contactsensors and methods of operating biometric skin contact sensors.

BACKGROUND

Secure, verifiable authentication, of user identity is an increasinglyimportant part of all technology. To give just a few examples, it playsa part in:

-   -   User equipment (UE) for communication and consumer access to        media content;    -   Computer devices and systems which store and provide access to        sensitive data;    -   Devices and systems for financial transactions or access control        to buildings; and    -   Access control for vehicles.

Biometric measurement of the user is now prevalent in all of thesecontexts and others. Biometric measures such as iris scanning, andfacial recognition are dependent on lighting and field of view of acamera. It may also be possible to circumvent such security measures bypresenting a video or photo of the user to the camera.

Fingerprint sensors have been thought of as being more secure, but it ispossible also to overcome the security they provide, and themanufacturing requirements of such sensors makes it difficult tointegrate them into other electronic devices such as mobile telephonesand other UEs. In particular, fingerprint sensing demands very highresolution—at least hundreds of pixels per inch.

One example of such a sensor is Apple Inc's Touch ID®. This sensor isbased on a laser-cut sapphire crystal. It uses a detection ring aroundthe sensor to detect the presence of the user's finger. The Touch ID®sensor uses capacitive touch sensing to detect the fingerprint, and hasa 500 pixel per inch (PPI) resolution.

Capacitance sensors such as these use capacitive effects associated withthe surface contours of the fingerprint. The sensor array pixels eachinclude an electrode which acts as one plate of a capacitor, the dermallayer (which is electrically conductive) acts as the other plate, andthe non-conductive epidermal layer acts as a dielectric. The capacitanceis greater where the dermis is closer to the pixel electrode, and so thesurface contours of the skin can be sensed by measuring the capacitanceof each pixel (e.g. based on the charge accumulated on the pixelelectrode) and assembling an image from those pixels.

Both passive matrix and active matrix capacitive touch sensors have beenproposed. Most so-called passive capacitive touch sensing systems use anexternal driving circuit (such as an integrated circuit, IC) to drive amatrix of passive electrodes, and a separate readout circuit (e.g. anIC) to readout charge stored on these electrodes during the drive cycle.The stored charge varies dependent on the tiny capacitance changes dueto touch events. Passive electrode systems are sensitive toenvironmental noise and interference.

Active matrix capacitive touch sensors include a switching element ineach pixel. The switching element may control a conduction path betweenthe capacitive sensing electrode in the pixel, and an input channel toan analogue to digital converter (ADC) in a read-out circuit. Typicallyeach column of pixels in an active array is connected to one such inputchannel. The charge stored in the array can thus be read from the activematrix by controlling the switching elements to connect each row ofpixels, one-by-one, to the ADC.

Each pixel needs to be connected to the read-out circuit, and all of thepixels of each column are effectively connected in parallel. Theparasitic capacitance associated with each pixel therefore combinesadditively. This places an inherent limit on the number of pixels thatcan be combined together in any one column. This in turn limits the sizeand/or resolution of a capacitive touch sensor.

There thus remains a significant unmet commercial need for large areahigh resolution touch sensors.

SUMMARY

Aspects of the disclosure are set out in the independent claims andoptional features are set out in the dependent claims. Aspects of thedisclosure may be provided in conjunction with each other, and featuresof one aspect may be applied to other aspects.

In an aspect, there is provided a capacitive biometric skin contactsensor configured to resolve the contours of skin in contact with thesensor. The sensor comprises: an array of sensor pixels, wherein eachsensor pixel comprises a thin film transistor and a capacitive sensingelectrode connected to the thin film transistor; a plurality of gatedrive channels, wherein each gate drive channel is arranged to provide agate drive signal to one or more of the sensor pixels; a plurality ofread-out channels, wherein each read-out channel is arranged to receivea read-out current from one or more of the sensor pixels, each read-outcurrent being indicative of a proximity to a respective capacitivesensing electrode of a conductive object to be sensed; a currentmultiplexer connected to a plurality of the read-out channels to receiveread-out currents therefrom; and a current mirror assembly connected tothe multiplexer to receive an input current therefrom and to provide aselected gain to the input current. The sensor is configured to controlboth: (i) the number of read-out currents selected by the multiplexer,and (ii) the selected gain to the input current.

Embodiments may enable the sensor to provide increased flexibility forcontrolling operational parameters of the sensor. In particular,embodiments may facilitate dynamic control of resolution and/orsensitivity for the sensor, as well as dynamic control of otherparameters such as total measurement times and energy consumption forthe sensor. For example, embodiments may enable control of resolution(e.g. by controlling the number of read-out currents selected by themultiplexer) and/or sensitivity (e.g. by controlling the selected gainto the input current). This may enable the sensor to be operatedaccording to one or more different operational parameters by controllingoperation of one or more components of the sensor. The sensor may beconfigured to control the selected gain based on (e.g. proportional to)the number of read-out currents selected by the multiplexer. Forexample, the sensor may be configured to control at least one of: (i)the selected gain, (ii) the number of currents selected by themultiplexer, and (iii) an amount of time for which gate-drive signalsare applied to a pixel and/or the number of pixels to which gate drivesignals are applied based on another one or more of these parameters.For example, this control of one or more parameters may be proportionalto said one or more other parameters.

Controlling the number of read-out currents selected by the multiplexermay comprise at least one of: (i) selecting one of the read-out currentsas an input current for the current mirror assembly, (ii) combining twoor more of the read-out currents to provide an input current for thecurrent mirror assembly, and (iii) multiplexing two or more of theread-out currents to provide a (time-ordered) sequence of input currentsfor the current mirror assembly (e.g. a first input current followed bya second input current). Multiplexing may comprise providing read-outcurrents, or combinations of read-out currents, as the input for thecurrent mirror assembly in a time-ordered sequence (e.g. it may comprisetime multiplexing, such as so that the input for the current mirrorassembly is indicative of different read-out currents at differentperiods in time).

The sensor may be configured to control the selected gain based on atleast one of: (i) the number of read-out currents selected by themultiplexer, and (ii) an amount of time for which the gate drive signalis applied to the one or more respective pixels. The sensor may beconfigured to increase the selected gain when the number of read-outcurrents selected decreases. For example, the sensor may be configuredto increase the selected gain when the number of read-out currentscombined to provide an input current for the current mirror assemblydecreases. For example, the sensor may be configured to decrease theselected gain when the amount of time for which the gate drive signal isapplied to the one or more respective pixels increases. For example, thesensor may be configured to control the selected gain so that, for eachsensor pixel measurement, the total amount of charge brought about bythat measurement (e.g. input current multiplied by amount of time)remains within a selected range. In other words, the sensor may beconfigured to operate the current mirror assembly to provide increasedgain where the charge brought about by that measurement decreases (andvice-versa). The sensor may be operated to provide a selected gain whichis based on properties of the sensor and/or pixels themselves. Forexample, the selected gain may be based on one or more of: the pixelsize, properties of the capacitive sensing electrode, a duration of gatedrive signals.

The sensor may be configured to control the number of read-out currentsselected by the multiplexer based on at least one of: (i) the selectedgain, and (ii) the amount of time for which the gate drive signal isapplied to the one or more respective pixels. For example, the sensormay be configured to reduce the number of read-out currents which arecombined when the selected gain is higher, and to increase the number ofread-out currents which are combined when the selected gain is lower.The sensor may be configured to increase the number of combined read-outcurrents, or reduce the number of individually selected read-outcurrents when the amount of time decreases (and vice-versa).

The sensor may be configured to control both: (i) the number of read-outcurrents selected by the multiplexer, and (ii) the selected gain, toprovide a selected level of resolution and/or sensitivity for thesensor. The sensor may be configured to decrease the number of differentread-out currents combined by the multiplexer and/or to increase theselected gain to increase the resolution and/or sensitivity for thesensor. For example, the number of different read-out currents combinedby the multiplexer (and also e.g. the number of individual read-outcurrents passed on as the input for the current mirror assembly) may beselected to provide a desired level of resolution, and/or the selectedgain may be controlled to provide a desired level of sensitivity foreach measurement.

The multiplexer may comprise a plurality of switches for controllingwhich of the read-out currents are selected. The sensor may beconfigured to select one or more values for the selected gain to theinput current. The current mirror assembly may comprise a plurality offield effect transistors. The biometric sensor may be configured tocontrol the selected gain to the input current by selecting which of thefield effect transistors are used. The current mirror assembly maycomprise: a first field effect transistor connected to the input currentat both its gate region and its drain region; and a plurality of secondfield effect transistors each connected to the input current at theirgate region and configured to output a scaled current at their sourceregion. For example, the second field effect transistors may beconfigured to provide a gain of greater than one (e.g. to provide anamplified current) or to provide a gain of less than one (e.g. toprovide a reduced current). The outputs from each of the second fieldeffect transistors may be connected to provide an output current fromthe current mirror assembly. Each of the second field effect transistorsmay have at least one different property to provide different amounts ofgain to the input current. The second field effect transistors may havedifferent width to length ratios. The current mirror assembly maycomprise a plurality of switches for controlling which of the secondfield effect transistors output an amplified current. The drain regionof each of the second field effect transistors may be connected to asupply voltage via a respective switch. The biometric sensor may beconfigured to selectively open or close said switches to control whichof the second field effect transistors output an amplified current.

The biometric sensor may comprise a plurality of multiplexers asdisclosed herein. Each of the plurality of read-out channels may becoupled to a said multiplexer to provide their respective read-outcurrent to said multiplexer. The multiplexer may be provided in anintegrated circuit. The current mirror assembly may be provided in anintegrated circuit. The multiplexer and the current mirror assembly maybe provided in the same integrated circuit (e.g. they may be provided ina single integrated circuit). Alternatively, the multiplexer may beprovided in the TFT array (e.g. the array of TFTs which provides thesensor array). For example, the multiplexer may be provided on the samesubstrate as the TFT array. The multiplexer may be provided on glassand/or foil (e.g. with the TFTs of the sensor array). The sensor may beconfigured to control which, and how many, of the read-out channelscoupled to the multiplexer provide their respective read-out current tothe multiplexer as its input at any given time. The current mirrorassembly may be configured to provide amplification of the current inputto the current mirror assembly from the current multiplexer to providean output current. The sensor may be configured to control operation ofthe current mirror assembly to select an amount of amplification itprovides to the multiplexer input. The biometric sensor may comprise anintegrator configured to integrate the output current from the currentassembly to provide an output voltage indicative of the proximity to theone or more respective capacitive sensing electrodes of the conductiveobject to be sensed. The output voltage (e.g. from the current mirrorassembly) may be provided to an analog-to-digital converter to determinethe proximity to one or more capacitive sensing electrodes of theconductive object to be sensed.

In an aspect, there is provided a method of operating a biometric sensorto resolve the contours of skin in contact with the sensor. The sensorcomprises: an array of sensor pixels, wherein each sensor pixelcomprises a thin film transistor and a capacitive sensing electrodeconnected to the thin film transistor; a plurality of gate drivechannels, wherein each gate drive channel is arranged to provide a gatedrive signal to one or more of the sensor pixels; a plurality ofread-out channels, wherein each read-out channel is arranged to receivea read-out current from one or more of the sensor pixels, each read-outcurrent being indicative of a proximity to a respective capacitivesensing electrode of a conductive object to be sensed; a currentmultiplexer connected to a plurality of the read-out channels to receiveread-out currents therefrom; and a current mirror assembly connected tothe multiplexer to receive an input current therefrom and to provide aselected gain to the input current. The method comprises: controllingboth: (i) the number of read-out currents selected by the multiplexer,and (ii) the selected gain to the input current.

The method may comprise receiving a control signal indicating a desiredlevel of resolution and/or sensitivity for the biometric sensor, andcontrolling both: (i) the number of read-out currents selected by themultiplexer, and (ii) the selected gain to the input current, to providethe desired level of resolution and/or sensitivity. The method maycomprise increasing the resolution and/or sensitivity by decreasing thenumber of read-out currents selected and/or increasing the selectedgain. The method may comprise controlling the selected gain based on atleast one of: (i) the number of read-out currents selected by themultiplexer, and (ii) an amount of time for which the gate drive signalis applied to the one or more respective pixels. The method may compriseincreasing the selected gain when the number of read-out currentsselected decreases.

In an aspect, there is provided a capacitive biometric skin contactsensor configured to resolve the contours of skin in contact with thesensor. The sensor comprises: an array of sensor pixels, wherein eachsensor pixel comprises a thin film transistor and a capacitive sensingelectrode connected to the thin film transistor; a plurality of gatedrive channels, wherein each gate drive channel is arranged to provide agate drive signal to one or more of the sensor pixels; a plurality ofread-out channels, wherein each read-out channel is arranged to receivea read-out current from one or more of the sensor pixels, each read-outcurrent being indicative of a proximity to a respective capacitivesensing electrode of a conductive object to be sensed; and an analog todigital converter comprising a dual slope integrator arranged toreceive, as its input, either an output current or a reference voltage,wherein said output current is based on one or more read-out currents.The biometric sensor is configured to: apply the output current as theinput to the dual slope integrator for a charging time period to chargea capacitor of the dual slope integrator; apply the reference voltage asthe input to the dual slope integrator for a discharging time period,wherein the discharging time period comprises the amount of time ittakes for the capacitor to discharge; and determine the proximity to theone or more respective capacitive sensing electrodes of the conductivebody based on an indication of the discharging time period.

Embodiments may enable the sensor to provide increased flexibility forcontrolling operational parameters of the sensor. In particular,embodiments may facilitate dynamic control of sensitivity for thesensor, as well as dynamic control of other parameters such as totalmeasurement times and energy consumption for the sensor. For example,embodiments may enable control of sensitivity (e.g. by controlling thecharging time period). This may enable the sensor to be operatedaccording to one or more different operational parameters by controllingoperation of one or more components of the sensor (e.g. the chargingtime period of the dual slope integrator). The sensor may be configuredto control the charging time period based on (e.g. proportional to) anamount of time for which gate-drive signals are applied to a pixeland/or the number of pixels to which gate drive signals are applied. Thedual slope integrator may be configured to switch between receiving theoutput current and receiving the reference voltage (e.g. to controlwhether the capacitor is charging or discharging).

The biometric sensor may be configured to control the charging timeperiod. For example, the sensor may be configured to increase ordecrease the charging time period. The biometric sensor may beconfigured to increase the charging time period to increase thesensitivity of the sensor (or vice-versa).

The sensor may comprise a current multiplexer connected to a pluralityof the read-out channels to receive read-out currents therefrom. Thebiometric sensor may be configured to control the number of read-outcurrents selected by the multiplexer. The biometric sensor may beconfigured to: control the number of read-out currents selected by themultiplexer based on the charging time period; and/or control thecharging time period based on the number of read-out currents selectedby the multiplexer. The sensor may comprise a current mirror assemblyconfigured to receive an input current indicative of the one or moreread-out currents and to provide a selected gain to the input current toprovide the output current. The sensor may be configured to control theselected gain to the input current. The biometric sensor may beconfigured to: control the selected gain to the input current based onthe charging time period; and/or control the charging time period basedon the selected gain to the input current.

The sensor may comprise: a current multiplexer connected to a pluralityof the read-out channels to receive read-out currents therefrom; and acurrent mirror assembly connected to the multiplexer to receive an inputcurrent therefrom and to provide a selected gain to the input current toprovide the output current. The sensor may be configured to control atleast one of: (i) the number of read-out currents selected by themultiplexer, and (ii) the selected gain to the input current.

Embodiments may enable the sensor to provide increased flexibility forcontrolling operational parameters of the sensor. In particular,embodiments may facilitate dynamic control of resolution and/orsensitivity for the sensor, as well as dynamic control of otherparameters such as total measurement times and energy consumption forthe sensor. For example, embodiments may enable control of resolution(e.g. by controlling the number of read-out currents selected by themultiplexer) and/or sensitivity (e.g. by controlling the selected gainto the input current and/or by controlling the charging time period).This may enable the sensor to be operated according to one or moredifferent operational parameters by controlling operation of one or morecomponents of the sensor. The sensor may be configured to control theselected gain based on (e.g. proportional to) the number of read-outcurrents selected by the multiplexer. For example, the sensor may beconfigured to control at least one of: (i) the selected gain, (ii) thenumber of currents selected by the multiplexer, (iii) the charging timeperiod, and (iv) an amount of time for which gate-drive signals areapplied to a pixel and/or the number of pixels to which gate drivesignals are applied based on another one or more of these parameters.For example, this control of one or more parameters may be proportionalto said one or more other parameters.

The sensor may be configured to control at least one of: (i) the numberof read-out currents selected by the multiplexer, and (ii) the selectedgain to the input current, based on the charging time period. The sensormay be configured to control at least one of: (i) the number of read-outcurrents selected by the multiplexer, and (ii) the charging time period,based on the selected gain to the input current. The sensor may beconfigured to control at least one of: (i) the selected gain to theinput current, and (ii) the charging time period, based on the number ofread-out currents selected by the multiplexer. The sensor may beconfigured to control at least one of: (i) the number of read-outcurrents selected by the multiplexer, and (ii) the selected gain to theinput current, to increase the output current when the charging timeperiod is decreased. The sensor may be configured to select theresolution and/or sensitivity of the sensor by controlling at least oneof: (i) the number of read-out currents selected by the multiplexer,(ii) the selected gain to the input current, and (iii) the charging timeperiod.

The dual slope integrator may comprise an integrator and a comparator.For example, the integrator may comprise an operational amplifierintegrator. The capacitor may be part of the integrator. The positiveterminal of the comparator may be connected to a reference voltage suchas ground. The negative terminal of the comparator may be connected tothe output from the integrator. The output from the comparator may beconnected to a controller configured to determine the discharging timeperiod. The dual slope integrator may comprise a reset switch connectedin parallel with the capacitor (e.g. the reset switch may be part of theintegrator).

The dual slope integrator may be arranged to receive, as its input,either an output current or a reference voltage. The output current maybe based on a read-out current from one or more of the read-outchannels. Each read-out current may be indicative of a proximity to arespective capacitive sensing electrode of a conductive object to besensed. The sensor may be configured to apply the output current as theinput to the dual slope integrator for a charging time period beforeapplying the reference voltage as the input to the dual slope integratoruntil the output from the dual slope integrator returns to zero (e.g.until a capacitor of the dual slope integrator discharges). The sensormay be configured to determine the proximity to the one or morerespective capacitive sensing electrodes of the conductive body based onthe length of time for which the reference voltage is applied to thedual slope integrator before the output from the dual slope integratorreturns to zero (e.g. based on the discharging time period). Thebiometric sensor may comprise a controller configured to control theselective application of the output current or the reference voltage asthe input to the dual slope integrator. The controller may be configuredto determine the length of time for which the reference voltage isapplied to the dual slope integrator before the output from the dualslope integrator returns to zero (e.g. the controller may be configuredto measure the discharging time, such as to enable an indication of theoutput current to be determined therefrom).

The reference voltage input for the dual slope integrator may compriseboth a lower reference voltage and a higher reference voltage. Forexample, the analog to digital converter may comprise a dual slopeintegrator arranged to receive, as its input, either an output current,a lower reference voltage or a higher reference voltage. The sensor maybe configured to switch the input to the dual slope integrator between:(i) the output current, (ii) the lower reference voltage, and (iii) thehigher reference voltage. The sensor may be configured to: use the lowerreference voltage as the input to the dual slope integrator to provide alonger discharging time period for a given charging time period, and/orto use the higher reference voltage as the input to the dual slopeintegrator to provide a shorter discharging time period for a givencharging time period. The sensor may be configured to select which ofthe reference voltages to use as the input to the dual slope integratorbased on a desired operational parameter for the sensor. For example,the sensor may be configured to use the lower reference voltage as theinput to provide a higher sensitivity measurement (e.g. to use the lowerreference voltage as the input when operating in a higher sensitivitymode) and vice-versa. For example, the sensor may be configured to usethe higher reference voltage as the input to provide a quickerdischarging time period, and thus a quicker measurement time period. Thesensor may be configured to use the higher reference voltage to providea quicker measurement time and/or a higher resolution measurement (e.g.to enable more measurements to be obtained in a given time period). Inan aspect, there is provided a method of operating a capacitivebiometric skin contact sensor configured to resolve the contours of skinin contact with the sensor. The sensor comprises: an array of sensorpixels, wherein each sensor pixel comprises a thin film transistor and acapacitive sensing electrode connected to the thin film transistor; aplurality of gate drive channels, wherein each gate drive channel isarranged to provide a gate drive signal to one or more of the sensorpixels; a plurality of read-out channels, wherein each read-out channelis arranged to receive a read-out current from one or more of the sensorpixels, each read-out current being indicative of a proximity to arespective capacitive sensing electrode of a conductive object to besensed; and an analog to digital converter comprising a dual slopeintegrator arranged to receive, as its input, either an output currentor a reference voltage, wherein said output current is based on one ormore read-out currents. The method comprises: applying the outputcurrent as the input to the dual slope integrator for a charging timeperiod to charge a capacitor of the dual slope integrator; applying thereference voltage as the input to the dual slope integrator for adischarging time period, wherein the discharging time period comprisesthe amount of time it takes for the capacitor to discharge; anddetermining the proximity to the one or more respective capacitivesensing electrodes of the conductive body based on an indication of thedischarging time period.

The method may comprise controlling the charging time period. The methodmay comprise increasing the charging time period to increase thesensitivity of the sensor. The biometric sensor may comprise a currentmultiplexer connected to a plurality of the read-out channels to receiveread-out currents therefrom. The method may comprise: controlling thenumber of read-out currents selected by the multiplexer based on thecharging time period; and/or controlling the charging time period basedon the number of read-out currents selected by the multiplexer. Thesensor may comprise a current mirror assembly configured to receive aninput current indicative of the one or more read-out currents and toprovide a selected gain to the input current to provide the outputcurrent. The method may comprise: controlling the selected gain to theinput current based on the charging time period; and/or controlling thecharging time period based on the selected gain to the input current.

The sensor may comprise: (i) a current multiplexer connected to aplurality of the read-out channels to receive read-out currentstherefrom; and (ii) a current mirror assembly connected to themultiplexer to receive an input current therefrom and to provide aselected gain to the input current to provide the output current. Themethod may comprise: controlling the resolution, sensitivity and/orsensing time of the sensor by controlling at least one of: (i) thenumber of read-out currents selected by the multiplexer, (ii) theselected gain to the input current, and (iii) the charging time period.The method may comprise controlling at least one of: (i) the number ofread-out currents selected by the multiplexer, and (ii) the selectedgain to the input current, based on the charging time period. The methodmay comprise controlling at least one of: (i) the number of read-outcurrents selected by the multiplexer, and (ii) the selected gain to theinput current, to increase the output current when the charging timeperiod is decreased. The method may comprise controlling operation of atleast one of: (i) the multiplexer, (ii) the current mirror assembly,and/or (iii) the dual slope integrator, based on an amount of time forwhich the gate drive signal is applied to each gate drive channel.

Embodiments of the present disclosure may comprise a sensor arraycomprising an array of sensor pixels. Each sensor pixel may comprise areference capacitor, a thin film transistor and a capacitive sensingelectrode. For each sensor pixel, the reference capacitor and thecapacitive sensing electrode are connected to a gate region of the thinfilm transistor. For each sensor pixel: the reference capacitor may beconnected in series with the capacitive sensing electrode so that, inresponse to a control voltage, an indicator voltage is provided at theconnection between the reference capacitor and the capacitive sensingelectrode to indicate a proximity to the capacitive sensing electrode ofa conductive object to be sensed; and the thin film transistor maycomprise a sense voltage-controlled impedance having a control terminalconnected so that the impedance of the sense voltage-controlledimpedance is controlled by the indicator voltage. Each sensor pixel maycomprise a reset circuit for setting the control terminal of the sensevoltage-controlled impedance to a reset voltage selected to tune thesensitivity of the pixels.

For example, aspects of the present disclosure provide a sensor arraycomprising a plurality of touch sensitive pixels, each pixel comprising:a capacitive sensing electrode for accumulating a charge in response toproximity of a conductive object to be sensed; a reference capacitorconnected in series with the capacitive sensing electrode so that, inresponse to a control voltage, an indicator voltage is provided at theconnection between the reference capacitor and the capacitive sensingelectrode to indicate the proximity of the conductive object to besensed. This arrangement may reduce or overcome the problem associatedwith parasitic capacitance which may occur in prior art touch sensors.

Each pixel may comprise a sense VCI (voltage controlled impedance)having a control terminal connected so that the impedance of the senseVCI is controlled by the indicator voltage. Typically the sense VCIcomprises at least one TFT (thin film transistor) and the conductionpath of the VCI comprises the channel of the TFT. A conduction path ofthe sense VCI may be connected to a first plate of the referencecapacitor, and the control terminal of the first VCI is connected to thesecond plate of the reference capacitor. At least one plate of thereference capacitor may be provided by a metallisation layer of a thinfilm structure which provides the sense VCI.

The conduction path of the sense VCI may connect the first plate of thereference capacitor, and so also the control voltage, to an input of areadout circuit. This may enable the circuitry which provides thecontrol voltage also to provide the basis for the output signal of thepixel. This may further address problems associated with parasiticcapacitance and signal to noise ratio in prior art touch sensors. Analternative way to address this same problem is to arrange theconduction path of the sense VCI to connect a reference signal supply toan input of a readout circuit. The reference signal supply may comprisea constant voltage current source. Thus, modulating the impedance of thesense VCI of a pixel controls the current from that pixel to the inputof the read-out circuit.

A select VCI may also be included in each pixel. This may be connectedso that its conduction path is connected in series between theconduction path of the sense VCI and the reference signal supply. Thus,switching the select VCI into a non-conducting state can isolate thesense VCI from the reference signal input, whereas switching the selectVCI into a conducting state can enable current to flow through the pixel(depending on the impedance of the sense VCI). A control terminal of theselect VCI may be connected for receiving the control voltage, e.g. froma gate drive circuit.

Each pixel may comprise a gate line VCI, and a conduction path of thegate line VCI may connect the reference signal supply to the first plateof the reference capacitor for providing the control voltage.

Each pixel may comprise a reset circuit for setting the control terminalof the sense VCI to a selected reset voltage. The reset circuit maycomprise a reset VCI. A conduction path of the reset VCI is connectedbetween a second plate of the reference capacitor and one of (a) a resetvoltage; and (b) a first plate of the reference capacitor. A controlterminal of the reset VCI may be connected to another pixel of thesensor for receiving a reset signal (e.g. from a channel of a gate drivecircuit which is connected to the control terminal of the select VCI ofa pixel in another row of the array). The reset signal may be configuredto switch the reset VCI into a conducting state, thereby to connect thesecond plate of the reference capacitor to the one of (a) the resetvoltage and (b) the first plate of the capacitor. Connecting the secondplate of the reference capacitor to the one of (a) the reset voltage.

In an aspect, there is provided a computer program product comprisingcomputer program instructions configured to program a controller toperform any method disclosed herein.

FIGURES

Some examples of the present disclosure will now be described, by way ofexample only, with reference to the figures, in which:

FIG. 1 shows a schematic diagram of exemplary processing circuitryincluding a current multiplexer and a current mirror assembly.

FIG. 2 shows a schematic diagram of an exemplary dual slope integrator.

FIG. 3 shows a schematic diagram of a portion of exemplary read-outcircuitry for a capacitive biometric skin contact sensor.

FIG. 4 comprises a plan view of a sensor apparatus comprising a sensorarray, and Inset A of FIG. 4 shows a circuit diagram for a pixel of thesensor array.

FIG. 5 shows a circuit diagram of a sensor array for a sensor apparatussuch as that illustrated in FIG. 4 .

FIG. 6 shows a circuit diagram of another sensor array of the type shownin FIG. 4 .

In the drawings like reference numerals are used to indicate likeelements.

SPECIFIC DESCRIPTION

Embodiments of the present disclosure provide circuitry for processingread-out currents from sensor pixels of a capacitive biometric skincontact sensor. In embodiments, a current multiplexer may be provided toselect which, and how many, read-out currents are to be measured at thesame time. For example, in a high resolution mode, the currentmultiplexer may be configured to time-multiplex all of its read-outcurrents in order (e.g. one-by-one). In embodiments, a current mirrorassembly may be provided to select an amount of gain to be provided toread-out currents to be measured. The current mirror assembly maycomprise a current scaler (e.g. a current amplifier). The current mirrormay be operable to provide scaled read-out currents to an analog todigital converter, such as a dual slope integrator analog to digitalconverter. In embodiments, such a dual slope integrator may be providedfor measuring read-out currents, where a charging time of the dual slopeintegrator may be varied. One or more of these components (the currentmultiplexer, the current mirror and/or the dual slope integrator) of thecircuitry for processing read-out currents may be controlled to providea selected amount of resolution and/or sensitivity for the biometricskin contact sensor. For example, one or more of: (i) the number ofread-out currents which are selected (by the current multiplexer), (ii)the selected gain (by the current mirror assembly), and (iii) theintegration time (of the dual slope integrator), may be controlled toprovide a selected amount of resolution and/or sensitivity for thebiometric skin contact sensor. Sensors of the present disclosure may beconfigured to control two or more of these components of the circuitry,and the control of each component may be selected based also on controlof the other component, e.g. so that the sensor as a whole providesdesired characteristics for resolution and/or sensitivity.

One example of circuitry for processing read-out currents from sensorpixels of a capacitive biometric skin contact sensor will now bedescribed with reference to FIG. 1 .

FIG. 1 shows processing circuitry 100. Processing circuitry 100 includesa current multiplexer 110 and a current mirror assembly 120. Also shownis a sensor array 10.

The current multiplexer 110 comprises a plurality of multiplexerswitches 111, 112, 113 and 114. Each switch is connected to a respectiveone out of a plurality of read-out channels 11, 12, 13 and 14 from thesensor array 10. The current mirror assembly 120 comprises a firsttransistor 121 and a plurality of second transistors 122, 123, 124 and125. Each of the second transistors is connected to a corresponding oneof a plurality of current mirror switches 126, 127, 128 and 129. Each ofthe second transistors is connected to the first transistor 121 so thatthe gate-source voltage of that second transistor mirrors thegate-source voltage of the first transistor. The conduction path of eachof the second transistors is connected in series with a correspondingone of a plurality of the plurality of current mirror switches. Each ofthe multiplexer switches is switchable between connecting itscorresponding read-out channel to ground or connecting its correspondingread-out channel to the current mirror assembly input.

Each multiplexer switch is connected to a respective corresponding oneof the plurality of read-out channels (e.g. a first multiplexer switch111 is connected to a first read-out channel). Each of the multiplexerswitches is also connected to a reference voltage (in the example shown,the reference voltage is ground). Each multiplexer switch is configuredto switch between being connected to ground and to providing aconnection between a read-out channel and the current mirror. Amultiplexer output line is connected to each of the read-out channelsdownstream of the switch. The multiplexer output line is connected tothe current mirror assembly 120 to provide an input current thereto.Each read-out current may pass through to ground (if the switch isclosed) or pass through to the current multiplexer 110 (if the switch isopen).

The multiplexer output line is connected to the current mirror assembly120 to provide an input current to the current mirror assembly 120. Inthe example shown in FIG. 1 , each transistor is an N Channel MOSFET.The multiplexer output line is connected to both the gate region and thedrain region of the first transistor 121 (the drain of the firsttransistor 121 is shorted to its gate). The source region of the firsttransistor 121 is connected to ground. The gate region of the firsttransistor 121 and the multiplexer output line are also connected to thegate region of each of the second transistors. The second transistorshave different width to length ratios. Each of the second transistors isconnected at their respective drain regions to receive a supply voltageV_(DD). The drain region of each second transistor is connected to itssupply voltage V_(DD) via a respective current mirror switch. The drainregion of each second transistor will receive the supply voltage V_(DD)if its respective switch is closed, and will not receive the supplyvoltage V_(DD) if its respective switch is open. The source region ofeach second transistor is connected to a current mirror assembly outputline. The current mirror assembly output line may be connected todownstream components for processing of current output from theplurality of second transistors.

The current multiplexer 110 is arranged to receive a plurality ofread-out currents from the array of sensor pixels. Each read-out channelof the array 10 is arranged to receive a read-out current from one ormore sensor pixels and to transmit said current to the multiplexer 110.For example, each read-out channel may be connected to a column ofsensor pixels to receive one or more read-out currents therefrom. Eachread-out channel has an associated multiplexer switch. The multiplexer110 is connected to a plurality of read-out channels, wherein each ofsaid read-out channels is connected to a respective switch of themultiplexer 110.

Each of the multiplexer switches is configured to selectively couple itsrespective read-out channel to ground or to the current mirror assembly120. Each multiplexer switch may be configured to operate in a firstmode in which a current from its read-out channel is directed to ground(the switch is closed), and in a second mode in which a current from itsread-out channel is directed to the current mirror assembly 120 (theswitch is open). Each of the read-out channels may receive read-outcurrents simultaneously, or at least within a very short time frame ofone another. The multiplexer 110 is arranged to enable selection of theread-out currents. The multiplexer 110 is configured to control which,and how many, of the read-out currents should be passed to the currentmirror assembly 120 at one time. That is, the multiplexer 110 isoperable to selectively open or close the multiplexer switches to enablea number of selected read-out currents to be provided to the currentmirror assembly 120. For example, the multiplexer 110 may simultaneouslyconnect read-out currents to the current mirror assembly 120 to combinethem (e.g. to provide read-out currents 1 to 4 to the current mirrorassembly 120 at the same time). As another example, the multiplexer 110may connect each individual read-out current to the current mirrorassembly 120 sequentially in time (e.g. read-out current 1, thenread-out current 2, then read-out current 3, then read-out current 4).

The multiplexer 110 is arranged to enable one read-out current to passto the current mirror assembly 120 at a time. To do this, themultiplexer 110 is controlled so that all but one of the multiplexerswitches are in their first mode. The read-out current associated withthe multiplexer switch in its second mode will then pass to the currentmirror assembly 120.

The multiplexer 110 is arranged to enable read-out currents frommultiple read-out channels to be combined (e.g. addition of currents).To do this, the multiplexer 110 is controlled so that two or moremultiplexer switches are in their second mode. The combined current willthen be passed to the current mirror assembly 120.

The multiplexer 110 is arranged to enable read-out currents frommultiple read-out channels to be multiplexed. To do this, themultiplexer 110 is controlled so that the switches are openedsequentially to provide multiplexing (time multiplexing). For example,the multiplexer 110 may be configured to enable one multiplexer switchto be in its second mode for a selected time period, while the remainingswitches are in their first mode, then the multiplexer 110 is controlledso that a different multiplexer switch is the switch in its second mode(thereby to provide a time-multiplexed current to the current mirrorassembly 120.

The multiplexer 110 is arranged to provide an input current to thecurrent mirror assembly 120, wherein that input current is based on oneor more read-out currents from read-out channels connected to themultiplexer 110. It will be appreciated that the input current providedmay vary depending on operation of the multiplexer 110. Where themultiplexer 110 selects only one read-out current to be provided to thecurrent mirror assembly 120 at any one time, the input current providedto the current mirror assembly 120 will correspond to that read-outcurrent (e.g. it will be that read-out current). Where the multiplexer110 combines two or more read-out currents, the input current providedto the current mirror assembly 120 will correspond to the combination ofthose read-out currents (e.g. it will be their sum). Where themultiplexer 110 multiplexes the read-out currents, the input currentprovided to the current will correspond to the individual read-outcurrent, or combination of read-out currents as part of the multiplexedcurrent at that time (e.g. it will be an individual read-out currentwhen multiplexing one current at a time, or a combined read-out currentwhen multiplexing multiple currents at a time).

The multiplexer 110 is therefore configured to control the input currentprovided to the current mirror assembly 120 by controlling operation ofthe switches (and thus controlling which of the read-out currents areselected). The current mirror assembly 120 is configured to receive, asits input, said current from the multiplexer 110.

Each of the second transistors of the current mirror assembly 120 isarranged to selectively receive a supply voltage V_(DD) at its drainregion. That is, each second transistor of the current mirror assembly120 is connected to the supply voltage V_(DD) by its respective currentmirror switch. Each current mirror switch is configured to operate in afirst mode in which no supply voltage V_(DD) is supplied to its drainregion and in a second mode in which the supply voltage V_(DD) issupplied to its drain region. The gate region of each of the secondtransistors is connected to the input current from the currentmultiplexer 110 and the gate region of the first transistor 121. Each ofthe second transistors is arranged to output a current at theirrespective drain region based on the input to their gate and drainregions respectively. It will be appreciated that in the absence of anyinput current being provided to the current mirror assembly 120, or anysupply voltage V_(DD) being provided to the drain region of a transistor(e.g. because the respective switch is in its first mode), there will beno output from the source region.

The current mirror assembly 120 is configured to receive the inputcurrent and to provide an output current based on the input current. Thecurrent mirror assembly 120 is configured to provide a selected scalingto the input current. That is, the current mirror assembly 120 isconfigured to provide a selected amount of gain to the input current.The current mirror assembly 120 is configured to control the amount ofgain by selecting which of the second transistors provide a current outof their source region. To do this, the current mirror assembly 120 isconfigured to control operation of each of the current mirror switches(whether each of the current mirror switches are open or closed). Bycontrolling operation of the current mirror switches, the current mirrorassembly 120 is thereby configured to control which of the secondtransistors receive an input at their respective drain regions and thuswhich provide an output at their respective source regions (assumingthere is an input current provided to the current mirror assembly 120).

Each of the second transistors may be configured to provide a differentoutput (at their source region). The second transistors are arranged toenable the current mirror assembly as a whole to provide a plurality ofdifferent amounts of gain to the input current. The second transistorsare configured so that, in response to the same input being provided toeach of the gate and drain regions, the output from the differenttransistors will be different. Each transistor is configured to providea different amount of gain to the input current. In this example, toprovide different amounts of gain, each transistor has a different widthto length ratio. The different width to length ratios are selected toprovide a numerical sequence for the gain ratios (e.g. a binary sequencesuch as 0.5, 1, 2, 4 may be used).

The current mirror assembly 120 is configured so that each of the secondtransistors will provide an output current in the event that itsrespective current mirror switch is in its second mode (closed) and aninput current is provided to the current mirror assembly 120. Thecurrent mirror assembly output line is configured to receive the outputcurrent from each of the second transistors. The total output currentfrom the current mirror assembly 120 will be the combination of theoutput currents from each of the individual second transistors. Thecurrent mirror assembly 120 is configured to provide a selected amountof gain to the input current by selecting which, and how many, of thesecond transistors are to provide an output current. For example, thecurrent mirror assembly 120 may be configured to identify that thedesired amount of gain corresponds to a combination of the secondtransistor, such as two, three or four of the second transistors (e.g.the desired amount of gain may correspond to the first and third of thesecond transistors 122, 124, and the switches for these transistors maybe closed while the switches for the remaining second transistors areopen).

In operation, the sensor array 10 is operated to provide one or moreread-out currents through the read-out channels. To do this, a gatedrive signal is applied to one or more of the sensor pixels of thesensor array 10. Each of the sensor pixels comprises a capacitivesensing electrode and a thin film transistor connector the capacitivesensing electrode. In response to receiving a gate drive signal, eachsensor pixel is configured to provide a read-out current which providesan indication of a proximity to said capacitive sensing electrode of aconductive object to be sensed. Said read-out currents are received byread-out channels which provide the read-out current to the currentmultiplexer 110. The sensor array 10 may be a rectangular grid of sensorpixels. A gate drive signal may be sequentially applied to each row inturn, with the gate drive signal being simultaneously applied to eachpixel in a row. Each time a gate drive signal is applied, a read-outcurrent may be obtained from all of the selected pixels in a column(e.g. this may be some, but not all of the pixels in a column, or all ofthe pixels in that column). These read-out currents are carried onread-out channels and provide the input to the current multiplexer 110.

Controlling operation of the processing circuitry 100 comprisescontrolling operation of the current multiplexer 110 and controllingoperation of the current mirror assembly 120. Operation of the currentmirror assembly 120 is controlled based on operation of the currentmultiplexer 110 (and vice-versa). Controlling operation of themultiplexer 110 comprises controlling which, and how many, of thecurrent multiplexer switches are open at any one time. Controllingoperation of the current mirror assembly 120 comprises controllingwhich, and how many, of the current mirror switches are open at any onetime. Controlling operation of the current multiplexer 110 willinfluence the magnitude of the input current provided to the currentmirror assembly 120, and what said input current represents. Controllingoperation of the current mirror assembly 120 will influence themagnitude of the output current from the current mirror assembly 120,and how that output current corresponds to the input current.

Operation of the current multiplexer 110 and current mirror assembly 120may be controlled to vary at least four operational parameters of theprocessing circuitry 100.

The first parameter is a resolution of the sensor array 10. Theresolution of the sensor array 10 comprises a spatial resolution foroperation of the sensor. This provides an indication of the smallestresolvable unit of the sensor array 10 (e.g. the spatial density ofunits from which sensor measurements have been obtained). The resolutionof the sensor array 10 is linked to the density of sensor pixels in thesensor array 10. The maximum resolution will correspond to the densityof sensor pixels in the sensor array 10. The resolution at any one timewill also be based on the extent of current multiplexing, as this willinfluence the density of units from which sensor measurements areobtained.

The second parameter is a sensitivity for sensor measurements of thesensor array 10. Sensitivity of the sensor array 10 provides anindication of how much the sensor output changes in dependence on howmuch the sensor input changes. For example, the sensitivity of thesensor array 10 may provide an indication of how accurately the obtainedmeasurement (sensed capacitance) corresponds to the true value for themeasured property (capacitance). Increasing sensitivity of a sensor maycomprise increasing the signal to noise ratio for obtained measurements.

The third parameter is a measurement time for sensor measurements of thesensor array 10. The measurement time provides an indication of thetotal amount of time it takes the sensor to obtain and process ameasurement (e.g. to perform capacitive biometric skin-contact sensingusing the sensor pixels of the sensor array 10). The measurement time islinked to the amount of time for which a gate drive signal is applied toeach sensor pixel (e.g. to each row of sensor pixels). The measurementtime will be based on the total amount of time taken to enable eachsensor pixel array 10 to have had one gate drive signal applied thereto(or at least the total amount of time taken to enable each sensor pixelto which a gate drive signal is to be applied to have a gate drivesignal applied thereto). For example, the minimum measurement time maybe the total amount of time taken to enable each sensor pixel to which agate drive signal is to be applied to have a gate drive signal appliedthereto (but it may be more than this, such as to allow signalprocessing time and/or to allow gate-signals to be applied to somepixels more than once).

The fourth parameter is the amount of energy required to obtain a sensormeasurement. That is, the amount of energy required to both obtain andprocess sensor measurements from each of the sensor pixels to which agate drive signal is to be applied.

It will be appreciated in the context of the present disclosure thatoperation of the sensor array 10 will influence these four parameters.Likewise, operating the sensor to provide a selected value for one ofthe parameters will influence (e.g. place constraints on) the selectedvalues for the other parameters. For example, to increase resolution,the multiplexer 110 may select each read-out current individually as theinput current to the current mirror assembly 120. This will provideincreased resolution but may also increase the measurement time andenergy consumption (for constant sensitivity) or decrease thesensitivity (for constant measurement time and energy consumption).

Embodiments of the present disclosure provide processing circuitry 100which enables these parameters to be varied. The circuitry 100 isconfigured to enable these parameters to be varied by controlling atleast one of: (i) operation of the current multiplexer 110, (ii)operation of the current mirror assembly 120, and (iii) the amount oftime for which each gate drive signal is applied to a sensor pixel.Operation of the sensor to control these properties will now bedescribed.

In operation, the resolution of the sensor may be controlled bycontrolling operation of the multiplexer 110 and/or the number of gatedrive signals applied to sensor pixels per each sensor cycle (one runthrough all the pixels from which measurements are to be obtained).

To increase the resolution of a sensor measurement, more individualread-out currents will have to be measured per sensor cycle. To do this,gate drive signals may be applied to more sensor pixels to ensure thatthere are more read-out currents for processing. For example, in a lowerresolution mode, gate drive signals may be applied to some, but not allof the sensor pixels (e.g. some rows of sensor pixels may be skipped),and in a higher resolution mode, gate drive signals may be applied tomore (e.g. all) of the sensor pixels (e.g. a gate drive signal may beapplied to each row of sensor pixels, such as, a gate drive signal maybe applied to each row in turn, e.g. applied row-by-row to one row at atime).

Where the number of gate drive signals applied to sensor pixels is heldconstant, the resolution of the sensor may be varied by controllingoperation of the current multiplexer 110. To increase the resolution,the current multiplexer 110 may be operated so that each input currentprovided to the current mirror assembly 120 is representative of asmaller region of pixels. For example, where four read-out currents arecombined into one input current for the current mirror assembly 120,this input current will correspond to a resolution which is less thanthat which would result if each read-out current was passed individuallyto the current mirror assembly 120. Therefore, to provide the highestresolution, the current multiplexer 110 may be operated to sequentiallyselect each read-out current to be provided to the current mirrorassembly 120 as the input current.

To do this, the multiplexer 110 is operated so that each multiplexerswitch connects its read-out channel to the current mirror assembly 120for a selected time period while the other three switches connect theirread-out channel to ground. For example, one switch may be open (toconnect its read-out channel to the current mirror assembly 120), andthe other switches may be closed (to respectively connect their read-outchannels to ground). As such, a first read-out current from the firstread-out channel 11 is provided to the current mirror assembly 120 asthe input current for the selected time period (e.g. multiplexer switch111 is open, with multiplexer switches 112, 113 and 114 closed). Then, asecond read-out current from the second read-out channel 12 is providedto the current mirror assembly 120 as the input current for the selectedtime period. Then, the same happens for a third read-out current, andthen for a fourth read-out current. The signal provided to the currentmirror assembly 120 over this entire time period may be considered to bea time-multiplexed signal where the input current provided to thecurrent mirror assembly 120 for each selected time period represents aread-out current from a different one of the pixels.

In operation, the multiplexer 110 may be controlled according to aselected level of resolution. Operation of the switches of themultiplexer 110 may correspond to this selected level of resolution. Alowest resolution may be provided where only one input current isprovided to the current mirror assembly 120 (e.g. either one individualread-out current or a combination of two or more read-out currents). Ahighest resolution may be provided where each individual read-outcurrent is provided separately to the current mirror assembly 120 as theinput current (in a time-sequential manner). Resolutions between thelowest and highest resolution may be provided where more than onedifferent input current is provided to the current mirror assembly 120,but fewer input currents are provided than the total number that wouldbe provided if one read-out current were provided as an input currentfor each of the read-out currents. For example, the sensor may operatein a plurality of different resolution modes. These resolution modes mayinclude a lowest resolution mode, a highest resolution mode and one ormore resolution modes corresponding to resolutions between the lowestand highest mode. The sensor may be configured to control operation inthe different resolution modes so that more read-out currents aretime-multiplexed in higher resolution modes, e.g. fewer read-outcurrents are combined in the higher resolution modes than in the lowerresolution modes.

As one example of controlling the resolution, read-out currents may bereceived at each of the four read-out channels. The multiplexer 110 isoperated to provide a medium level of resolution. To do this, themultiplexer 110 operates so that the first and second multiplexerswitches 111, 112 are initially open, and the third and fourthmultiplexer switches 113, 114 are shut (as shown in FIG. 1 ) so that acombination of current passing along the first and second read-outchannels 11, 12 is provided as the input current to the current mirrorassembly 120 and current passing along third and fourth read-outchannels 13, 14 flows to ground. Thus, a first measurement is processedwhich corresponds to one measurement obtained from the two pixels whichprovided a read-out current to the first and second read-out channels11, 12. Then, the multiplexer switches are changed so that the first andsecond multiplexer switches 111, 112 are closed and the third and fourthmultiplexer switches 113, 114 are opened. Then, a combination of currentpassing along the third and fourth read-out channels 13, 14 is providedas the input current to the current mirror assembly 120 and currentpassing along first and second read-out channels 11, 12 flows to ground.Thus, a second measurement is processed which corresponds to onemeasurement obtained from the two pixels which provided a read-outcurrent to the third and fourth read-out channels 13, 14. This processis then repeated as read-out currents are received on the read-outchannels which correspond to different sensor pixels in the array 10.

In operation, the sensitivity of the sensor may be controlled bycontrolling operation of the current multiplexer 110, the current mirrorassembly 120 and/or the amount of time for which gate drive signals areapplied to sensor pixels per each sensor cycle.

The length of time for which gate drive signals are applied to sensorpixels may be selected to provide the desired level of sensitivity (e.g.sensitivity may be increased by applying a gate drive signal to eachsensor pixel for a longer period of time, and decreased by applying agate drive signal for a shorter period of time). The length of time forwhich the multiplexer 110 passes one or more read-out currents as theinput current to the current mirror assembly 120 may be controlled tovary sensitivity (e.g. if each input current to the current mirrorassembly 120 is provided for a longer period of time the sensitivity mayincrease as compared to if it were provided for a shorter period oftime). Measuring the signals downstream of the current mirror assembly120 may comprise use of one or more integrator. It is to be appreciatedin the context of the present disclosure that the signal to noise ratioof an integrated measurement may be improved with longer integrationtime. The number of read-out currents combined by the multiplexer 110may also be controlled to provide a selected sensitivity.

Operation of the current mirror assembly 120 may be controlled to varythe sensitivity of measurements obtained by the sensor. To provideincreased sensitivity for a given measurement, the current mirrorassembly 120 may be operated to provide greater gain to the inputcurrent, and to provide less sensitivity for a given measurement, thecurrent may be operated to provide less gain to the input current.Operation of the current mirror assembly 120 may be controlled toprovide a selected amount of gain to the input current (e.g. based on adesired sensitivity level for the sensor).

For the current mirror assembly 120 to provide the maximum amount ofgain to the input current, all of the second transistors will beoperated to provide an output current. This will provide the greatesttotal output amount of gain. That is, this will provide the greatestoutput current for a given input current. The current mirror assembly120 will be controlled so that each of the current mirror switches areclosed (so that the supply voltage V_(DD) is delivered to the drainregion of each of the second transistors, and thus each of the secondtransistors may provide an output current from their source region).

For the current mirror assembly 120 to vary the amount of gain, thenumber of switches that are closed, and which of the switches it is thatare closed is selected. For a given input current to the current mirrorassembly 120, each of the second transistors will provide a differentoutput current. Providing a selected gain to the input current by thecurrent mirror assembly 120 comprises selecting the gain to be providedand identifying one second transistor, or a combination of secondtransistors, required to provide said gain. The current mirror switchesare then controlled so that the selected one or more of the secondtransistors provide output current (e.g. have their respective currentmirror switches closed) and the remaining second transistors to do notprovide output current (e.g. have their respective current mirrorswitches open). The current output from the one second transistor with aclosed switch, or the combination of currents output from the secondtransistors with closed switches, then provides the output current fromthe current mirror assembly 120 which will have the selected gain fromthe input current. In the example shown in FIG. 1 , the fourth currentmirror switch 129 is closed and the rest are open. The gain to the inputcurrent will be that provided by the fourth second transistor 125.

In operation, the measurement time of the sensor may be controlled bycontrolling the amount of time for which a gate drive signal is appliedto each sensor pixel to which a gate drive signal is to be applied,and/or by controlling the amount of sensor pixels to which a gate drivesignal is to be applied. For one sensor cycle, the measurement time willcorrespond to the amount of time it takes to apply the gate drive signalto each pixel (or row of pixels) in the array 10 to which a gate drivesignal is to be applied. The sensor may therefore reduce the measurementtime by reducing the amount of time for which each gate drive pulse isapplied and/or by applying the gate drive signal to fewer sensor pixels.The sensor may control these factors to provide a desired measurementtime.

In operation, the energy consumption of the sensor may be controlled bycontrolling operation of the current multiplexer 110, the current mirrorassembly 120, and optionally the amount of time for which each gatedrive signal is applied, and/or the amount of sensor pixels to which agate drive signal is applied per each sensor cycle. To decrease theenergy consumption of the sensor, the multiplexer 110 may be operated totake fewer measurements of different read-out currents. For example, themultiplexer may select only one read-out current to reduce energyconsumption associated with generating and processing additionalread-out currents. The sensor may also operate to combine a greaternumber of read-out currents to reduce energy consumption (for a fixednumber of read-out currents available), thereby to provide an inputcurrent to the current mirror assembly 120 which has a greatermagnitude. To decrease the energy consumption of the sensor, the currentmirror assembly 120 may be operated to provide less gain. To decreasethe energy consumption of the sensor, gate drive signals may be appliedfor less time, or gate drive signals may be applied to fewer sensorpixels. One or more of these parameters may be controlled to provide aselected amount of energy consumption for the sensor.

In addition to controlling operating of one component of the sensor toprovide a selected value for the parameters described above, multiplecomponents of the sensor may be controlled in combination. One componentof the sensor may be controlled to compensate for undesirable effects onone or more of these parameters brought about by controlling operationof the sensor to improve another of these parameters.

Operation of the current mirror assembly 120 may be controlled tocompensate for operation of the current multiplexer 110.

That is, the current multiplexer 110 is configured to control the numberof the read-out currents to be selected. Where the current multiplexer110 combines more read-out currents, the magnitude of the input currentprovided to the current mirror assembly 120 is likely to be greater(this will depend on the proximity to each respective capacitive sensingelectrode of the sensor array 10 of a conductive body). As such, thecurrent mirror assembly 120 may be operated to provide less gain to thisinput current. Where the current multiplexer 110 selects individualread-out currents as the input current to the current mirror assembly120, the magnitude of this input current is likely to be less. As such,the current mirror assembly 120 may be operated to provide more gain tothis input current.

The current mirror assembly 120 may be controlled to provide a selectedamount of gain to the input current, where that selected gain isdetermined based on the number of read-out currents selected by thecurrent multiplexer 110. The number and selection of current mirrorswitches which are closed may be selected based on the number of currentmultiplexer switches which are open.

In the event that the current multiplexer 110 is being operated in amanner expected to provide a greater input current to the current mirrorassembly 120, the current mirror assembly 120 is operated to provideless gain to the input current. In the event that the currentmultiplexer 110 is being operated in a manner expected to provide asmaller input current to the current mirror assembly 120, the currentmirror assembly 120 is operated to provide more gain to the inputcurrent. The current mirror assembly 120 may be operated to provide anamount of gain to the input current selected based on an expected valuefor the input current.

Operation of the current mirror assembly 120 may be controlled tocontrol the measurement sensitivity for a given resolution, as providedby the operation of the current multiplexer 110. That is, where thecurrent multiplexer 110 is being operated to obtain higher resolutionmeasurements (e.g. where a greater number of different input currentsare provided to the current mirror assembly 120 per unit time, and wherethese input currents are typically of smaller magnitude), then thecurrent mirror assembly 120 is operated to provide a greater increase insensitivity for these measurements (by providing greater gain).Likewise, where the current multiplexer 110 is being operated to obtainlower resolution measurements (e.g. where fewer different input currentsare provided to the current mirror assembly 120 per unit time, and wherethese input currents are typically of greater magnitude, e.g. becausetwo or more read-out currents have been combined to provide said inputcurrents), then the current mirror assembly 120 is operated to provideless of an increase in sensitivity for these measurements (by providingless gain).

Operation of the current mirror assembly 120 may also be controlledbased on the amount of time for which each gate drive signal is appliedto a pixel. The current mirror assembly 120 may be controlled to provideincreased gain to the input current when the amount of time for whicheach gate drive signal is applied to a pixel decreases. The currentmirror assembly 120 may be configured to provide increased gain to theinput current when the gate drive signal is applied to fewer pixels(e.g. this may be to compensate for the fact that the multiplexer 110may then only be able to combine fewer read-out currents). For example,the current mirror assembly 120 may provide more gain to allow forreduced gate drive time and/or increased resolution.

In a similar manner, operation of the current multiplexer 110 may becontrolled to compensate for operation of the current mirror assembly120. That is, the current mirror assembly 120 may be configured toprovide a certain amount of gain to the input current. Operation of themultiplexer 110 may be controlled so that the input current provided tothe current mirror assembly 120 is of a suitable current for operationat the selected gain. For example, where the current mirror assembly 120may be operating at a lower value for the selected gain, the multiplexer110 may combine more read-out currents.

Operation of the current multiplexer 110 may be controlled based on thenumber of pixels to which gate drive signals are applied and/or theamount of time for which each gate drive signal is applied. The currentmultiplexer 110 may obtain an indication of which of its read-outchannels should receive a read-out current from a sensor pixel. Thecurrent multiplexer 110 may be controlled based on this obtainedindication, such as to prevent a read-out channel being selected(especially on its own) for which no read-out current is expected. Thecurrent multiplexer 110 may be controlled so that the fewer the amountof pixels from which a read-out current is received, the fewer differentinput currents the multiplexer 110 may provide to the current mirrorassembly 120.

The current multiplexer 110 may be controlled so that the amount of timefor which it selects read-out currents for the input current iscontrolled based on the amount of time for which a gate drive signal isapplied to the pixels (e.g. to ensure that each read-out current ispassed to the current mirror assembly 120 as, or as part of, the inputcurrent in that time period). The current multiplexer 110 may beconfigured to select the number of different input currents to beprovided to the current mirror assembly 120 for one gate drive signalbased on the amount of time for which the gate drive signal is applied.For example, the multiplexer 110 may be controlled so that the less theamount of time for which the gate drive signal is applied, the fewerdifferent input currents the multiplexer 110 may provide to the currentmirror assembly 120.

It will be appreciated in the context of the present disclosure thatoperation of the circuitry 100 shown in FIG. 1 enables increased freedomto select and implement different operational parameters for the sensor.That is, for a given set of gate drive parameters, the currentmultiplexer 110 may be operated to vary the resolution for the sensor.For a given resolution (as controlled at least in part by the currentmultiplexer 110), the current mirror assembly 120 may be operated tovary the sensitivity of the sensor. Depending on input parameters forthe sensor, the current multiplexer 110 and current mirror assembly 120may be controlled to provide a selected resolution and sensitivity forthe sensor.

The output current from the current mirror assembly 120 may then bepassed onto downstream circuitry of the sensor which is arranged toprocess said output current, and to determine therefrom, an indicationof the proximity to each respective capacitive sensing electrode of aconductive object to be sensed. One example of components which may formpart of said downstream circuitry will now be described with referenceto FIG. 2 .

FIG. 2 shows a dual slope integrator 200. The dual slope integrator 200has an input 210 which includes a first input 211 and a second input212. The dual slope integrator also includes an integrator 250. Forexample, the integrator 250 may comprise an operational amplifierintegrator. The integrator 250 includes a resistor 215, an amplifier220, and a capacitor 222. Optionally, as shown in FIG. 2 , theintegrator 250 includes a reset switch 224. The dual slope integrator250 also includes a comparator 230. The dual slope integrator 200 alsoincludes a control unit 240, which includes a processor 242, and atiming element 244.

The input 210 to the dual slope integrator 200 comprises a switchconnection which is connectable to the first input 211 and the secondinput 212. The switch connection is arranged to switch between the firstand second input so that it will be connected to either the first input211 or the second input 212. The switch connection of the input 210 isconnected to the resistor 215 of the integrator 250. The resistor 215 isconnected to both the inverting input of the amplifier 220 and a firstplate of the capacitor 220. The resistor 215 is also connected to thereset switch 224. The non-inverting input of the amplifier 220 isconnected to a reference voltage, such as ground, as shown in FIG. 2 .The capacitor 222 is connected between the inverting input of theamplifier 220 and the output terminal of the amplifier 220. The resetswitch 224 is connected across the capacitor 224 (e.g. so that whenclosed, the switch shorts the plates of the capacitor 222).

The connection between the output terminal of the amplifier 220 and asecond plate of the capacitor 222 is connected to the negative input tothe comparator 230. The reset switch 224 is also connected to theconnection between the output terminal of the amplifier 220 and thesecond plate of the capacitor 222. The positive input to the comparator230 is connected to a reference voltage, such as ground (as shown inFIG. 2 ). The output from the comparator 230 is connected to the controlunit 240. The timing element 244 is connected to the processor 242.

The input 210 to the dual slope integrator 200 is arranged to switchbetween the first input 211 and the second input 212. The first input211 is arranged to receive a current indicative of one or more read-outcurrents. Each read-out current is indicative of a proximity to arespective capacitive sensing electrode of a conductive object to besensed. The second input 212 is arranged to receive a reference voltage.The input 210 is thus arranged to switch between the current indicativeof one or more read-out currents and the reference voltage. The input210 is arranged to be connected to the first input 211 for a chargingtime period. That is, the input 210 is arranged to receive the currentindicative of the one or more read-out currents for the charging timeperiod. The input 210 is arranged to then switch to the second input 212to receive the reference voltage. In this example, the reference voltageis negative (e.g. it has been inverted).

The dual slope integrator 200 is arranged so that the switch connectionof the input either connects the first input 211 or the second input 212to the inverting input of the amplifier 220 and the first plate of thecapacitor 222 via the resistor 215 (e.g. the switch connection mayswitch between connecting the first input 211 to the integrator 250 andconnecting the second input 212 to the integrator 250).

The integrator 250 is arranged so that, in the event that the switchconnection of the input 210 is connected to the first input 211 (e.g. toreceive the current indicative of one or more read-out currents), thecapacitor 222 charges up. The sensor is configured to control the switchconnection of the input 210 of the dual slope integrator 200 so that thecapacitor 222 will be charged up for the charging time period (by thecurrent indicative of the one or more read-out currents). The dual slopeintegrator 200 is arranged so that the amount of charge stored by thecapacitor 222 is indicative of: (i) the current indicative of the one ormore read-out currents, and (ii) the charging time period. Thus, thedual slope integrator 200 is arranged so that, when the charging timeperiod is held constant for subsequent charges (e.g. for subsequentcurrents indicative of one or more read-out currents), the amount ofcharge stored by the capacitor 222 for each charge will be indicative ofthe value of that current indicative of the one or more read-outcurrents. The reset switch 224 is arranged to be open during charging ofthe capacitor 222 (e.g. it does not provide a conductive path forcurrent to short the capacitor 222).

The integrator 250 is arranged so that, in the event that the switchconnection of the input 210 is connected to the second input 212 (e.g.to receive the reference voltage), the capacitor 222 discharges. Thesensor is configured to control the switch connection of the input 210of the dual slope integrator 200 so that the input 210 will be connectedto the second input 212 for enough time for the capacitor 222 to fullydischarge. For example, the dual slope integrator 200 may be arranged sothat the input 210 will remain connected to the second input 212 until areceived indication that the capacitor 222 has discharged (e.g. asprovided by the controller 240 monitoring the output of the comparator230).

The comparator 230 is arranged to receive, as its negative input, theoutput from the integrator 250. The dual slope integrator 200 isarranged so that, in the event that the capacitor 222 stores some charge(e.g. it is not fully discharged), the comparator 230 will have anegative voltage at its negative input. The comparator 230 is connectedto a reference voltage at its positive terminal, such as ground (asshown in FIG. 2 ). The comparator 230 is therefore arranged to provide afirst output signal in the event that the capacitor 222 is at leastpartially charged, and a second output signal in the event that thecapacitor 222 is fully discharged. The integrator 200 is arranged sothat the output from the comparator 230 provides an indication ofwhether the capacitor 222 is charged or discharged. That is, once thecharging time period has started and the discharging time period has yetto finish (and so there is at least some charge stored in the capacitor222), the output of the comparator 230 will indicate the presence of anegative voltage at its negative input. Once a discharging time periodis complete, and no charging time period has yet commenced (and so thecapacitor 222 is discharged), such as after a measurement has beenobtained, the output of the comparator 230 will indicate that there isno longer a negative voltage at its negative input (e.g. the voltage atits negative input is no longer below the voltage at its positive input,as they are both zero/grounded).

The control unit 240 is arranged to receive an indication of the outputfrom the second comparator 230. The processor 242 is arranged to receivea clock signal from the timing element 244. The processor 242 isconfigured to determine an indication of the amount of time for whichthe capacitor 222 is discharging (e.g. to determine the discharging timeperiod). That is, the processor 242 is configured to monitor the outputfrom the comparator 230 and to measure the amount of time (using clocksignals from the timing element 244) for which the output of thecomparator 230 indicates the presence of charge on the capacitor 222. Inthis example, that amount of time corresponds to the amount of time forwhich the negative input to the comparator 230 has a negative voltage.Once the output from the comparator 230 has tripped (e.g. it switches toindicate that its negative input is no longer less than its positiveinput), the controller 240 is configured to obtain an indication of whenthe output tripped (from the timing element 244) and to determine a timeperiod for the duration between the time when the output of thecomparator 230 indicated the capacitor 222 started charging and the timewhen the output of the comparator 230 indicated the capacitor 222 haddischarged. As the charging time period will be known, this combinedcharging and discharging time period will provide an indication ofdischarging time period.

The control unit 240 may be configured to start a timer when the input210 is first connected to the first input 211 (and thus when thecapacitor 222 starts charging). The control unit 240 is configured torun the timer until the output signal from the comparator 230 indicatesthat the capacitor 222 is discharged. The total time on the timer willinclude both the charging time period and a discharging time period(e.g. the charging time period may be subtracted from the total timeperiod to provide the discharging time period). The discharging timeperiod will be indicative of the amount of charge that was stored on thecapacitor 222 in the selected time period, and thus indicative of thevalue for the current indicative of the one or more read-out currents.

The reset switch 224 is arranged so that, when open, it will short thecapacitor 222. The dual slope integrator 200 may be arranged to closethe switch 224 after each measurement (e.g. to short the capacitor 222and thereby reset the dual slope integrator 200 for the subsequentmeasurement). For example, in the event that an indication that themeasurement has been taken is received, the dual slope integrator 200may be controlled to close the switch 224 and reset the dual slopeintegrator 200 for the next measurement (at which point the switch 224will then be opened).

The dual slope integrator 200 is arranged so that, based on: (i) anobtained indication of the discharging time period for the capacitor222, and (ii) the charging time period, the current indicative of theone or more read-out currents may be determined.

In operation, the timer is started, and the input 210 is connected tothe first input 211 (with the reset switch 224 open). The input 210remains connected to the first input 211 for the charging time period.During the charging time period, the first input 211 (e.g. from thecurrent indicative of the one or more read-out currents) charges thecapacitor 222. This brings about a negative voltage at the negativeinput to the comparator 230. This negative voltage increases inmagnitude as more time passes with the first input connected to theintegrator 250.

Then, after the charging time period has ended, the input 210 isswitched so that it is connected to the second input 212 (e.g. thenegative reference voltage). During this time period, the voltage at thenegative terminal of the comparator 230 is increased (e.g. to begin withthe magnitude of this voltage decreases as it returns towards zero froma negative value). The output from the comparator 230 is monitored bythe control unit 240. The output from the comparator 230 will indicatewhether or not the voltage at its positive input is greater than orequal to that at its negative input. As the voltage at the positiveinput is zero (ground), the output of the comparator 230 will changeonce the voltage at its negative input is no longer negative. Once theoutput from the comparator 230 changes (thus indicating that thecapacitor 222 has fully discharged), the timer is stopped. The amount ofcharge that was stored by the capacitor 222 (and thus an indication ofthe current indicative of the one or more read-out currents) isdetermined based on the discharging time period (as determined based onthe amount of time that passed before the output from the comparator 230tripped). The discharging time period will be proportional to the amountof charge stored by the capacitor 222 (and thus the current output fromthe sensor pixel). For example, for a higher current output, thedischarge time will be longer (e.g. increased as compared to a shortercharging time period or lower current output). The reset switch 224 isthen closed to reset the dual slope integrator 200 for the nextmeasurement.

The sensor is operable to vary the charging time period. That is, thesensor is configured to control the amount of time for which the input210 is connected to the first input 211 and the capacitor 222 is chargedby the current output from the sensor pixel. The sensor is configured toincrease the charging time period to increase the sensitivity of themeasurement. The system is configured to decrease the charging timeperiod to provide quicker measurement times. The system thus enables avariable trade-off between the sensitivity of measurements and theamount of time taken for one measurement to be obtained. For example,operation of the dual slope integrator 200 (its charging time period)can be controlled to provide a selected level of sensitivity ormeasurement time.

It is to be appreciated in the context of the present disclosure thatincreasing the amount of charge provided to the capacitor of anintegrator of an ADC of a sensor may provide an increased signal tonoise ratio. Such increasing of the charge on the capacitor may beachieved by combining more read-out currents (by the current multiplexer110), providing increased gain (by the current mirror assembly 120)and/or increasing the charging time period (of the dual slope integrator200). Over a longer charging period, more variables which may influencethe measurement value may have averaged out, such that fluctuations inthe measurement may be smoothed. As one example, the timing element 244may count in discrete time periods, and so any measurement of thedischarge time may be accurate to within one of said time periods. Bycharging the capacitor 222 for longer, the discharge time will also belonger, but the time period of the timing element 244 will be the same.Thus, the accuracy limit brought about by one time period of the timingelement 244 will be a smaller proportion of the total time measurement.As another example, to the extent that there may be any current orcharge in the sensor which is undesirably applied to the capacitor 222of the dual slope integrator 200 (e.g. a residual current/charge from aprevious measurement or brought about by other components of thesensor), the contribution of this charge to the total charge on thecapacitor 222 will be less if the charging time is increased, and thusthe overall charge on the capacitor 222 is increased.

The sensor may be arranged to have a lower limit for the charging time.The lower limit may correspond to a minimum level of sensitivity. Thelower limit may be selected based on an expected value (or values, suchas a range from a minimum to a maximum expected value) for the currentinput to the dual slope integrator 200 from the sensor pixels. Thesensor may be arranged to have an upper limit for the charging time. Theupper limit may correspond to a maximum acceptable measurement time. Theupper limit may correspond to a total capacity of the capacitor 222(e.g. to reduce the likelihood that the charging time period will belong enough for the capacitor 222 to fully charge). The upper limit maybe selected based on an expected value (or values, such as a range froma minimum to a maximum expected value) for the current input to the dualslope integrator 200 from the sensor pixels.

The sensor may be configured to select a value for charging time perioddepending on a desired level of sensitivity or measurement time for thesensor measurements. For example, the sensor may be configured to obtainan indication of a desired level of sensitivity or measurement time, andthe sensor may control the charging time period accordingly. The sensormay have a plurality of set values for the charging time period, andcontrolling the sensor based on the obtained indication may compriseselecting one of the plurality of values for the charging time period.For example, the plurality of values may range from the lower limit tothe upper limit. Alternatively, or additionally, the sensor may beconfigured to control the charging time period to any given time period(e.g. the charging time period may be a continuous variable which thesensor can control). The sensor may be configured to vary the chargingtime period for measurements from different pixels, or the charging timeperiod may be the same for each pixel.

As one example of operation of the sensor, the senor obtains anindication of a desired level of sensitivity for the measurementsindicative of a proximity to a respective capacitive sensing electrodeof a conductive object to be sensed. The sensor selects a charging timeperiod for the operation of the dual slope integrator 200 based on theobtained indication of the desired level of sensitivity. The higher thedesired level of sensitivity, the longer the charging time period. Theinput 210 is thus connected to the first input 211 for the selectedcharging time period. Then, the input 210 is switched to the secondinput 212, and the discharging time measured. For subsequentmeasurements, the charging time period may remain as this selectedcharging time period, or it may be controlled to be a different chargingtime period (e.g. to provide greater or lower sensitivity for othermeasurements).

In the example described above with reference to FIG. 2 , the referencevoltages (for the amplifier 220 and the comparator 230) are shown to beground. However, it will be appreciated in the context of the presentdisclosure that this should not be considered limiting. Other referencevoltages may be provided. In the event that the reference voltageconnected to the comparator 230 is not zero, it will be appreciated thatthe output from the comparator 230 may trip when the other input to thecomparator 230 is at a different value (e.g. when the voltage at thenegative input is no longer less than that at the positive input). Itwill also be appreciated that the circuitry may be inverted (e.g. sothat the current indicative of the one or more read-out currents isnegative). It will be appreciated that the output from the comparator230 may trip when the voltage at its negative input is no longer withergreater than, or less than, the voltage at its positive input. Thedischarging time period will provide an indication of the magnitude ofthe charge that was stored on the capacitor. For example, where thecharge on the capacitor is of greater magnitude, the magnitude of thedifference in voltage between that at the negative input to thecomparator 230 and the positive input to the comparator 230 will begreater, and vice versa. This greater difference in magnitude in thevoltages at the positive and negative inputs of the comparator 230 willbe evident in a greater discharging time period.

Processing circuitry described herein (e.g. the processing circuitry 100described above with reference to FIG. 1 ) may be combined with Dualslope integrators described herein (e.g. the dual slope integrator 200described above with reference to FIG. 2 ). FIG. 3 shows an example ofsuch an arrangement.

FIG. 3 shows a portion of a sensor 300 which includes the processingcircuitry 100 of FIG. 1 and the dual slope integrator 200 of FIG. 2 Theportion of the sensor 300 shown in FIG. 3 corresponds to a portion ofthe read-out circuitry configured to process read-out currents todetermine an indication of the proximity to the relevant capacitivesensing electrodes of the conductive body to be sensed. As shown, theprocessing circuitry 100 includes the current multiplexer 110 and thecurrent mirror assembly 120. The output of the current mirror assembly120 is connected to the dual slope integrator 200. The output from thecurrent mirror assembly 120 is connected to the first input 211 of theinput 210 of the dual slope integrator 200. That is, the sensor 300 isarranged so that the dual slope integrator 200 may selectively receive,at its first input 211, the output current from the current mirrorassembly 120.

The control unit 240 is connected to each of the current multiplexerswitches, the current mirror switches, and the dual slope integratorinput 210. The control unit 240 may also be connected to reset switch224 (if provided) for the dual slope integrator 200. These connectionsare shown in FIG. 3 as dashed lines. The control unit 240 is configuredto control operation of each of these components. That is, for each ofthe current multiplexer switches and/or the current mirror switches, thecontrol unit 240 is configured to control which of these switches areopen or closed, and for how long these switches are open or closed. Thecontrol unit 240 is configured to control to which of the first orsecond input the input 210 of the dual slope integrator 200 isconnected, and for how long this connection occurs. The control unit 240may also be configured to control whether the reset switch 224 is openor closed, and for how long this occurs.

The current multiplexer 110, the current mirror assembly 120 and thedual slope integrator 200 provide three variables of the sensor 300whose operation may be controlled by the control unit 240 to provide oneor more desired properties for measurements obtained by the sensor 300.The control unit 240 may control one or more of: (i) the number ofread-out currents selected by the current multiplexer 110 (as describedabove with reference to FIG. 1 ), (ii) the selected amount of gainprovided by the current mirror assembly 120 (as described above withreference to FIG. 1 ), and (iii) the charging time period for the dualslope integrator 200 (as described above with reference to FIG. 2 ). Itwill be appreciated in the context of the present disclosure that theseparameters may be controlled to vary the resolution, sensitivity and/ormeasurement time/energy consumption as described herein. It will furtherbe appreciated that operation any one (or two) of these components maybe controlled based on operation of the other one or more of thesecomponents. This functionality will now be described in more detail.

As described herein, controlling operation of the current multiplexer110 to select individual read-out currents may enable higher resolutionmeasurements to be obtained for the sensor 300. Controlling operation ofthe current multiplexer 110 to select individual read-out currents mayprovide a lower input current to the current mirror assembly 120 (ascompared to the current multiplexer 110 combining multiple read-outcurrents). Controlling operation of the current mirror assembly 120 toprovide increased gain to the input signal may enable higher sensitivitymeasurements to be obtained. Controlling operation of the current mirrorassembly 120 to provide increased gain may also increase energyconsumption for the sensor 300. Controlling operation of the dual slopeintegrator 200 so that the charging time period is longer may provideincreased sensitivity for the sensor 300. Controlling operation of thesensor 300 so that the charging time period is longer may increase thetotal measurement time for the sensor 300 and/or the energy consumptionof the sensor 300.

Examples of different ways to control these components of the sensor 300(and their associated resulting parameters) will now be described.

Resolution of the sensor 300 may be selected by controlling operation ofthe current multiplexer 110. As disclosed herein, to increaseresolution, the multiplexer 110 will combine fewer read-out currents,e.g. so that the total number of different input currents provided tothe current mirror assembly 120 increases. That is, each input currentprovided to the current mirror assembly 120 will either berepresentative of a smaller region of the sensor array 10, or more inputcurrents will be provided to the current mirror assembly 120 for thesensor array 10.

In the event that the current multiplexer 110 is operated to providehigher resolution, it may be expected that each individual input currentto the current mirror assembly 120 will be of a lower magnitude than ifmore read-out currents had been combined to provide said input current.The sensor 300 may be configured to compensate for this drop in inputcurrent brought about by increasing the resolution. To do this, thesensor 300 may increase the amount of gain provided by the currentmirror assembly 120 and/or increase the charging time period for thedual slope integrator 200.

The sensor 300 may be configured to control the increase in gainprovided by the current mirror assembly 120 and/or the charging timeperiod for the dual slope integrator 200 based on (e.g. proportional to)the change in resolution provided by the current multiplexer 110. Forexample, an expected change to the input current to current multiplexer110 associated with the increase in resolution may be determined. Theincrease in gain provided by the current mirror assembly 120 may beselected to be proportional to, or as close as possible to proportionalto, the expected drop in the input current brought about by the increasein resolution (e.g. the combination of second transistors used may beselected to provide a percentage increase in gain corresponding to thepercentage decrease in input current associated with the increasedresolution). The increase in charging time for the dual slope integrator200 may be selected to be proportional to the expected drop in the inputcurrent to the current mirror assembly 120. For example, the chargingtime may be selected so that the charge accumulated on the capacitor 222may be similar to that accumulated for the shorter charging time periodand lower resolution.

The sensor 300 may be configured to vary both the charging time periodand the selected gain to compensate for the decrease in input currentbrought about by the increased resolution. It is to be appreciated inthe context of the present disclosure that any suitable combination ofthe two may be provided. For example, the selected gain or charging timemay remain constant while the other variable is changed. As anotherexample, both the selected gain and the charging time may change. Thecombination of changing both parameters may be selected to provide adesired overall increase in sensitivity and/or an overall performanceefficiency e.g. a combination of one or more of sensitivity, resolution,measurement time and/or energy consumption.

The sensor 300 may be configured to determine how to control operationof the sensor 300 to provide the desired level of sensitivity based onone or more constraints associated with operation of the sensor 300. Forexample, the amount of time for which the gate drive signal is appliedto each gate-drive channel and/or the amount of time for which aread-out current is obtained from each read-out channel may set an upperlimit for the charging time period. Where read-out currents aremultiplexed (e.g. time multiplexed), the amount of time for which eachindividual read-out current is provided as the input current to thecurrent mirror assembly 120 may set an upper limit for the charging timeperiod. As another example, there may only be a finite number ofdifferent possible values for gain provided by the current mirrorassembly 120 (e.g. which correspond to different combinations of secondtransistors). Based on the intended resolution (e.g. based on anindication of the intended operation of the current multiplexer 110),the sensor 300 may determine a corresponding increase in gain and/orcharging time period. This may be determined based on stored data (e.g.a mapping between multiplexer operation parameters and correspondingoperation parameters for the current mirror assembly 120 and/or dualslope integrator 200), or this may be determined on-the-fly (e.g. basedon an obtained measurement indicative of the input current provided tothe current mirror assembly 120).

The sensor 300 may thus be configured to vary operation of the currentmirror assembly 120 and/or the dual slope integrator 200 based on theresolution of the sensor 300. In particular, measurement sensitivity maybe maintained by increasing the gain and/or the charging time periodwhen the resolution increases (and vice-versa).

Also, the sensor 300 may be configured to vary measurement sensitivityindependently of any changes in resolution. That is, the sensitivity ofthe sensor 300 may be increased or decreased for a constant or varyingresolution by controlling the current mirror assembly 120 and/or dualslope integrator 200 accordingly. In a manner similar to the exampledescribed above, sensitivity may be increased by increasing the chargingtime period and/or by providing more gain (and vice-versa). The sensor300 may obtain an indication of a desired level of sensitivity, and thesensor 300 may be configured to control operation of the current mirrorassembly 120 and the dual slope integrator 200 to provide said desiredlevel of sensitivity.

Operation of the sensor 300 (the current multiplexer 110, the currentmirror assembly 120 and/or the dual slope integrator 200) may becontrolled based on one or more alternative desired parameters for thesensor 300. The alternative desired parameters may comprise at least oneof: (i) a time for one sensor cycle (the total time taken to obtain asensor measurement from each of the pixels for which a measurement is tobe obtained), and (ii) an amount of energy consumption per sensormeasurement (either per pixel or per sensor cycle). The sensor 300 maybe configured to control operation of the sensor 300 based on thesealternative desired parameters by controlling the sensor 300 tocompromise (or compensate for) on one or more of sensitivity andresolution for the sensor 300. It will be appreciated that the highestresolution, highest sensitivity measurements will likely take longer andbe more energy consuming than lower resolution/sensitivity measurements.

To reduce the energy consumption, the sensor 300 may be configured to:(i) take fewer measurements (e.g. more read-out currents may becombined), (ii) provide less gain, and/or (iii) use a lower chargingtime period. For example, to keep energy consumption under a thresholdamount, the gain may be controlled to be at or below a threshold value.To compensate for reduced gain, and/or to reduce the number ofmeasurements which are needed (and total measurement time), the sensor300 may operate the current multiplexer 110 to combine more read-outcurrents. That way, the input current to the current mirror assembly 120will be higher (and so the output from the current mirror assembly 120may remain at a similar value to the output current had there been moregain and a lower input current). By combining read-out currents (andproviding fewer individual or multiplexed read-out currents to thecurrent mirror assembly 120), the energy consumption for the sensor 300may be reduced (in so doing, resolution may also decrease).Alternatively, or in addition, the charging time period may be reduced.This may reduce the amount of active time for the sensor 300 per sensorcycle, thereby reducing the energy consumption per sensor cycle.

To reduce the total measurement time, the number of measurements takenper sensor cycle may be decreased. To do this, the amount of gate drivesignals applied to gate drive channels and/or the amount of time forwhich each gate drive signal is applied may be reduced. The currentmultiplexer 110 may be configured to combine more read-out currents,and/or to skip read-out currents associated with pixels to which no gatedrive signal is applied. The current mirror assembly 120 may beconfigured to provide increased gain to the input current it receivesfrom the current multiplexer 110. It will be appreciated that themaximum charging time for the dual slope integrator 200 may decreasewhen the amount of time for which each gate drive signal is applieddecreases. The current multiplexer 110 may therefore combine moreread-out currents, and/or the current mirror assembly 120 may providemore gain to allow for the charging time decreasing.

The sensor 300 may therefore reduce the total energy consumption ormeasurement time while providing a maintained (or at least less reduced)sensitivity or resolution for sensor measurements. This trade-off may becontrolled by controlling operation of the current multiplexer 110,current mirror assembly 120 and/or dual slope integrator 200accordingly.

Thus, the sensor 300 provides components which enable their respectiveparameters to be adapted so as to provide desired operational parametersfor the sensor 300. That is, the current multiplexer 110, current mirrorassembly 120 and/or dual slope integrator 200 may be operatedaccordingly to vary the sensitivity, resolution, measurement timings andenergy consumption for the system. Each of these components may enablesome flexibility for controlling operational parameters of the sensor300. In addition to each of these individual components enabling thisflexibility in operational parameters of the sensor 300, thesecomponents may also operate together to provide even greaterflexibility. For example, one or more components may be controlled tocompensate for an operational parameter brought about by operatinganother one of the components in a particular way. These components mayalso enable one or more operational parameters of the sensor 300 to befixed to a desired level, with another operational parameter thencontrolled (e.g. maximised) according to the fixed parameter. Forexample, for a given limit on energy consumption or measurement time,the sensor 300 may be configured to compensate to provide the highestresolution or sensitivity possible for the given limit.

It is to be appreciated in the context of the present disclosure thatthe examples described above are exemplary but not limiting on the scopeof the claims. Instead, the above description provides examples of howthe technology may be implemented. However, this technology may beimplemented in alternative or additional ways. Also, features describedabove should not be considered as essential unless explicitly stated so,as these features may either not be provided, or alternative featuresmay be provided instead.

For example, it is to be appreciated that the exact arrangement ofswitches shown in the Figs. is not to be considered limiting. Instead,the arrangement shown is intended to illustrate the functionality ofswitching. For example, the switches of the current mirror assembly 120may instead selectively connect the input current to the gate region ofthe second transistors (rather than selectively connecting thetransistors to the supply voltage V_(DD)). As another example, whereground voltages are shown in the Figs., these could instead be providedby a reference voltage that is not grounded. For instance, the firsttransistor 121 may be connected to a reference voltage, and the negativeinput to the comparator and/or the non-inverting input to theoperational amplifier 220 may also be connected to the same referencevoltage (or a different reference voltage). The switches of the currentmultiplexer 110 could also be connected to such a reference voltage, oranother reference voltage.

In the examples described above, the current mirror assembly 120 may bearranged to provide one of a plurality of different selected values forgain. For this, the second transistors are described as having differentvalues for their width to length ratios. However, it is to beappreciated in the context of the present disclosure that this is notlimiting. For example, each second transistor could have the same widthto length ratio, and different values for gain could be provided byselectively combining their respective output currents. Alternatively,or additionally, a different property of these second transistors couldbe varied, such as the property of the material of the transistoritself. As another example, one or more resistors may be provided toenable each second transistor to provide a different output (e.g. one ormore current limiting resistors may separate the supply voltage V_(DD)from the second transistors/the second transistors from the currentmirror assembly output current, or one or more resistors may be locatedbetween the current input and the gate region of some or all of thesecond transistors).

In the examples described above, the current multiplexer 110 and thecurrent mirror assembly 120 may be provided in one integrated circuit.That is, the current multiplexer 110 and the current mirror assembly 120may be part of the same integrated circuit. However, this need not bethe case. The current multiplexer 110 and the current mirror assembly120 may be provided by separate integrated circuits. The currentmultiplexer 110 may be implemented as part of the sensor array (e.g. aspart of the TFT array for sensor pixels). This array (and current mirrorassembly 120) may be provided on a suitable substrate, such as glass ora flex substrate or foil.

In examples described herein, a dual slope integrator 200 may beprovided. The dual slope integrator 200 is arranged to receive theoutput current from the current mirror assembly 120, and to form part ofthe analogue to digital conversion circuitry which enables a digitalmeasurement to be obtained from the sensor array 10. However, it is tobe appreciated that the output from a current multiplexer 110 and/orcurrent mirror assembly 120 need not be provided to a dual slopeintegrator 200. For example, the output from the current mirror assembly120 (or current multiplexer 110 in examples without a current mirrorassembly 120) may be connected directly, or indirectly, to alternativecircuitry for analogue to digital conversion. Such circuitry may includealternative components such as an op amp integrator in combination witha sample and hold circuit. In examples which include a dual slopeintegrator 200, it will be appreciated that the current output from thecurrent mirror assembly 120 (or the current multiplexer 110 in exampleswithout a current mirror assembly 120) may be connected directly orindirectly (e.g. via additional components) to the input 210 to the dualslope integrator 200.

It is to be appreciated in the context of the present disclosure thateach read-out current may be received at the read-out channel for anamount of time corresponding to the amount of time for which a gatedrive signal is applied to the sensor pixel (and the amount of time forwhich read-out circuitry is operated to obtain a read-out current fromthat read-out channel). Operation of the multiplexer 110 may becontrolled so that any changes to the multiplexer switches occur duringthis time period (e.g. so that if a read-out current from eachindividual read-out channel is wanted, they are all measured during thistime period). For example, the sensor 300 may be controlled so that whenmore read-out currents are to be measured individually, the gate drivesignals are applied to the sensor pixels for a longer period of time.The current multiplexer 110 may be operated so that each individualread-out current is fed to the current mirror assembly 120 for the sameamount of time, or different timings may be used (e.g. to give more timeto a more important pixel).

In the examples described herein, a reset switch 224 is provided to thedual slope integrator 200, but it will be appreciated that the dualslope integrator 200 may function without said reset switch 224. A givenmeasurement may be obtained from the sensor 300 without the need for areset, and/or alternative circuitry may be provided which enablesresetting of the dual slope integrator 200.

In examples described herein the input 210 to the dual slope integrator200 is connected to a first input 211 or a second input 212, where thefirst input 211 is indicative of one or more read-out currents and thesecond input 212 is a reference voltage. However, the input to the dualslope integrator 200 may have additional inputs. For example, thearrangement may be inverted to enable the inputs to be provided to thealternate terminal of the comparator. As another example, additionalinputs may be provided. For example, a third input may be provided whichprovides a different value for the reference voltage. It is to beappreciated in the context of the present disclosure that the sensor 300may be configured to switch between the second and third input toprovide a selected reference voltage as the input (and thus provided aselected discharging rate for the dual slope integrator 200). The sensor300 may be configured to select between the second and third input toprovide desired discharge speeds (e.g. to control operational parameterssuch as sensitivity, total measurement time and energy consumption).

It is to be appreciated in the context of the present disclosure thatthe sensor 300 may be configured to control timing of gate drivesignals. For example, although not shown in FIG. 3 , the control unit240 may be connected to the gate drive circuit to control application ofgate drive signals therefrom. The gate drive signals may be controlledbased on operational parameters of the sensor 300 such as the intendedapplication of the current multiplexer 110 (e.g. how many read-outcurrents are to be selected), the current mirror assembly 120 (e.g. howmuch gain is to be applied) and/or the dual slope integrator 200 (e.g.the discharge time period). For example, longer gate drive signals maybe applied to each pixel where greater resolution and/or sensitivity iswanted (e.g. to enable more read-out currents to be multiplexed forgreater resolution and/or to enable a longer charging time period forgreater sensitivity). Alternatively, operation of the components of thesensor 300 may be controlled based on selected conditions for the gatedrive signals. For example, the charging time period may be selected tobe less than or equal to the gate drive signal application time, and/orthe number of read-out currents to be multiplexed and/or the amount ofgain provided by the current mirror assembly 120 may be controlled basedon the available time for read-out currents to be passed as inputcurrents to the current mirror assembly 120.

In examples described herein, sensors may comprise a sensor array 10including a plurality of sensor pixels, wherein each sensor pixel isconfigured to provide an indication of the proximity to that sensorpixel of a conductive object to be sensed. With reference to FIGS. 4 to6 , some exemplary arrangements for such sensor arrays and sensor pixeldesigns will now be described.

FIG. 4 shows a sensor apparatus 2001 including a sensor array 2010 whichmay provide sensor arrays of the present disclosure disclosed herein.FIG. 5 illustrates a circuit diagram of one such sensor array 2010. Thedescription which follows shall refer to FIG. 4 and FIG. 5 together. Itcan be seen from an inspection of FIGS. 4 and 5 that inset A of FIG. 4shows a detailed view of one pixel of this array 2010.

The sensor array 2010 comprises a plurality of touch sensitive pixels2012. Typically, other than in respect of its position in the array,each pixel 2012 is identical to the others in the array 2010. Asillustrated, each pixel 2012 comprises a capacitive sensing electrode2014 for accumulating a charge in response to proximity of the surfaceof a conductive object to be sensed. A reference capacitor 2016 isconnected between the capacitive sensing electrode 2014 and a connectionto a gate drive channel 2024-1 of a gate drive circuit 2024. Thus, afirst plate of the reference capacitor 2016 is connected to the gatedrive channel 2024-1, and a second plate of the reference capacitor 2016is connected to the capacitive sensing electrode 2014.

Each pixel 2012 may also comprise a sense VCI (voltage-controlledimpedance) 2020 having a conduction path, and a control terminal (2022;inset A, FIG. 4 ) for controlling the impedance of the conduction path.The conduction path of the sense VCI 2020 may connect the gate drivechannel 2024-1 to an output of the pixel 2012. The control terminal 2022of the VCI is connected to the capacitive sensing electrode 2014 and tothe second plate of the reference capacitor 2016. Thus, in response to acontrol voltage applied by the gate drive channel 2024-1, the referencecapacitor 2016 and the capacitive sensing electrode 2014 act as acapacitive potential divider.

The capacitance of the capacitive sensing electrode 2014 depends on theproximity, to the capacitive sensing electrode 2014, of a conductivesurface of an object to be sensed. Thus, when a control voltage isapplied to the first plate of the reference capacitor 2016, the relativedivision of that voltage between that sensing electrode 2014 and thereference capacitor 2016 provides an indication of the proximity of thesurface of that conductive object to the capacitive sensing electrode2014. This division of the control voltage provides an indicator voltageat the connection 2018 between the reference capacitor 2016 and thecapacitive sensing electrode 2014. This indicator voltage can be appliedto the control terminal 2022 of the sense VCI 2020 to provide an outputfrom the pixel 2012 which indicates proximity of the conductive object.

Pixels may be positioned sufficiently close together so as to be able toresolve contours of the skin such as those associated with epidermalridges, for example those present in a fingerprint, palmprint or otheridentifying surface of the body. It will be appreciated in the contextof the present disclosure that contours of the skin may comprise ridges,and valleys between those ridges. During touch sensing, the ridges maybe relatively closer to a sensing electrode than the “valleys” betweenthose ridges. Accordingly, the capacitance of a sensing electrodeadjacent a ridge will be higher than that of a sensing electrode whichis adjacent a valley. The description which follows explains how systemscan be provided in which sensors of sufficiently high resolution toperform fingerprint and other biometric touch sensing may be providedover larger areas than has previously been possible.

As shown in FIGS. 5 and 6 , in addition to the sensor array 2010, such asensor may also comprise a dielectric shield 2008 (which may provide acarrier material), a gate drive circuit 2024, and a read out circuit2026. A connector 2025 for connection to a host device may also beincluded. This may be provided by a multi-channel connector having aplurality of conductive lines. This may be flexible, and may comprise aconnector such as a flexi, or flexi-rigid PCB, a ribbon cable orsimilar, e.g. the connector 2025 may comprise a flex foil. The connector2025 may carry a connection point 2027 configured to enable connectionto a host interface, such as a plug or socket, for connecting theconductive lines in the connector to signal channels of a host device inwhich the sensor apparatus 2001 is to be included. Alternatively, theconnector 2025 may itself carry a host interface, or at least a portionthereof.

The connection point 2027 of the connector 2025 may enable a hostinterface to be connected to the read-out circuit 2026. A controller(2006; FIG. 6 ) may be connected to the gate drive circuit 2024 foroperating the sensor array, and to the read-out circuit 2026 forobtaining signals indicative of the self-capacitance of pixels of thesensor array 2010.

The dielectric shield 2008 is generally in the form of a sheet of aninsulating material which may be transparent and flexible such as apolymer or glass. The dielectric shield 2008 may be flexible, and may becurved. An ‘active area’ of this shield overlies the sensor array 2010.In some embodiments, the VCIs and other pixel components are carried ona separate substrate, and the shield 2008 overlies these components ontheir substrate. The shield 2008 may provide the substrate for thesecomponents.

The sensor array 2010 may take any one of the variety of forms discussedherein. Different pixel designs may be used, typically however thepixels 2012 comprise at least a capacitive sensing electrode 2014, areference capacitor 2016, and at least a sense VCI 2020.

The array illustrated in FIG. 5 comprises a plurality of rows of pixelssuch as those illustrated in FIG. 4 . Also shown in FIG. 5 is the gatedrive circuit 2024, the read out circuit 2026, and a controller 2006.The controller 2006 is configured to provide one or more controlsignals, such as a clock signal, e.g. a periodic trigger, to the gatedrive circuit 2024, and to the read-out circuit 2026.

The gate drive circuit 2024 comprises a plurality of gate drive channels2024-1, 2024-2, 2024-3, which it is operable to control separately, e.g.independently. Each such gate drive channel 2024-1, 2024-2, 2024-3comprises a voltage source arranged to provide a control voltage output.And each channel 2024-1 is connected to a corresponding row of pixels2012 of the sensor array 2010. In the arrangement shown in FIG. 5 eachgate drive channel 2024-1, 2024-2, 2024-3 is connected to the firstplate of the reference capacitor 2016 in each pixel 2012 of its row ofthe sensor array 2010. During each clock cycle, the gate drive circuit2024 is configured to activate one of the gate drive channels 2024-1,2024-2, 2024-3 by applying a gate drive pulse to those pixels. Thus,over a series of cycles the channels (and hence the rows) are activatedin sequence, and move from one step in this sequence to the next inresponse to the clock cycle from the controller 2006.

The read-out circuit 2026 comprises a plurality of input channels2026-1, 2026-2, 2026-3. Each input channel 2026-1, 2026-2, 2026-3 isconnected to a corresponding column of pixels 2012 in the sensor array2010. To provide these connections, the conduction path of the sense VCI2020 in each pixel 2012 is connected to the input channel 2026-1 for thecolumn.

Each input channel 2026-1, 2026-2, 2026-3 of the read out circuit 2026may comprise an analogue front end (AFE) and an analogue-to-digitalconverter (ADC) for obtaining a digital signal from the column connectedto that input channel 2026-1. For example it may integrate the currentapplied to the input channel during the gate pulse to provide a measureof the current passed through the sense VCI 2020 of the active pixel2012 in that column. The read out circuit 2026 may convert this signalto digital data using the ADC. Furthermore, the analogue front endperforms impedance matching, signal filtering and other signalconditioning and may also provide a virtual reference.

In the sensor array 2010 shown in FIG. 5 , the conduction channel of thesense VCI 2020 in each pixel connects the input channel of the read outcircuit for that column to the gate drive channel for the pixel's row.In FIG. 5 , the gate drive channel for the row thus provides a referenceinput. Operation of the sense VCI 2020 modulates this reference input toprovide the pixel output. This output signal from a pixel indicates thecharge stored on the capacitive sensing electrode 2014 in response tothat reference input relative to that stored on the reference capacitor.

FIG. 4 includes a grid as a very schematic illustration of the rows andcolumns of pixels 2012 which make up the array. Typically this will be arectilinear grid, and typically the rows and columns will be evenlyspaced. For example the pixels may be square. It will of course beappreciated that the grid shown in FIG. 4 is not to scale. Typically thesensor array has a pixel spacing of at least 200 dots per inch, dpi (78dots per cm). The pixel spacing may be at least 300 dpi (118 dots percm), for example at least 500 dpi (196 dots per cm).

Operation of the sensor array 2010 of FIG. 5 now be described.

On each cycle of operation, the gate drive circuit 2024 and the read outcircuit 2026 each receive a clock signal from the controller 2006. Thecontroller 2006 may also control a gate start pulse to be delivered,e.g. to allow scanning to start at the top of the array (e.g. at a firstgate-drive channel).

In response to this clock signal, the gate drive circuit operates one ofthe gate drive channels to apply a control voltage to one of the rows ofthe array. In each pixel in the row, the control voltage from the gatedrive channel is applied to the series connection of the referencecapacitor 2016 and the capacitive sensing electrode 2014. The voltage atthe connection 2018 between the two provides an indicator voltageindicating the proximity of a conductive surface of an object to besensed to the capacitive sensing electrode 2014. This indicator voltagemay be applied to the control terminal of the sense VCI 2020 to controlthe impedance of the conduction path of the sense VCI 2020. A current isthus provided through the conduction path of the sense VCI 2020 from thegate drive to the input channel for the pixel's column. This current isdetermined by the gate drive voltage, and by the impedance of theconduction channel.

In response to the same clock signal, the read-out circuit 2026 sensesthe pixel output signal at each input channel. This may be done byintegrating the current received at each input of the read-out circuit2026 over the time period of the gate pulse. The signal at each inputchannel, such as a voltage obtained by integrating the current from thecorresponding column of the array, may be digitised (e.g. using an ADC,such as by using at least one of: the current multiplexer 110, thecurrent mirror 120 and/or the dual slope integrator 200 disclosedherein). Thus, for each gate pulse, the read-out circuit 2026 obtains aset of digital signals, each signal corresponding to a column of theactive row during that gate pulse. So the set of signals togetherrepresent the active row as a whole, and the output from each pixelbeing indicative of the charge stored on and/or the self-capacitance ofthe capacitive sensing electrode 2014 in that pixel.

Following this same process, each of the gate-drive channels isactivated in sequence. This drives the sense VCI 2020 of each pixelconnected to that channel into a conducting state for a selected time(typically the duration of one gate pulse). By activating the rows ofthe array in sequence the read out circuit, can scan the sensor arrayrow-wise. Other pixel designs, other scan sequences, and other types ofsensor array, may be used.

FIG. 6 illustrates another sensor array which may be used in the sensorsdisclosed herein. For example, the sensor array of FIG. 6 may be used inthe sensor array 2010 illustrated in FIG. 4 .

FIG. 6 shows a sensor array 2010 comprising a plurality of pixels, and areference signal supply 2028 for supplying a reference signal to thepixels. This can avoid the need for the gate drive power supply also toprovide the current necessary for the read-out signal.

Also shown in FIG. 6 is the gate drive circuit 2024, the read-outcircuit 2026, and the controller 2006.

The sensor array 2010 may also benefit from the inclusion of a resetcircuit 2032, 2034 in each pixel. This may allow the control terminal2022 of the pixel to be pre-charged to a selected reset voltage whilstthe pixel is inactive (e.g. while another row of the array is beingactivated by the application of a gate pulse to another, different, rowof the array).

In these embodiments the sensor may also comprise a reset voltageprovider 2042 for providing a reset voltage to each of the pixels 2012of the array as described below. The reset voltage provider 2042 maycomprise voltage source circuitry, which may be configured to provide acontrollable voltage, and may be connected to the controller 2006 toenable the controller 2006 to adjust and fix the reset voltage.

The reset voltage may be selected to tune the sensitivity of the pixel.In particular, the output current of the sense VCI 2020 typically has acharacteristic dependence on the indicator voltage at the controlterminal 2022 and its switch-on voltage. Thus the reset voltage may bechosen based on the switch-on voltage of the sense VCI 2020. Thecharacteristic may also comprise a linear region in which it may bepreferable to operate.

The pixels illustrated in FIG. 6 are similar to those illustrated inFIGS. 3 and 4 in that each comprise a capacitive sensing electrode 2014,and a reference capacitor 2016 connected with a capacitive sensingelectrode 2014. The connection between these two capacitances providesan indicator voltage, which can for example be connected to the controlterminal 2022 of a sense VCI 2020. In addition, the pixels of the sensorarray illustrated in FIG. 6 also comprise a further two VCIs 2034, 2038,and a connection to the reset voltage provider 2042, and a connection tothe reference signal supply 2028.

As noted above, the sense VCI 2020 is arranged substantially asdescribed above with reference to FIG. 4 , in that its control terminal2022 is connected to the connection between the reference capacitor 2016and the capacitive sensing electrode 2014. However, the conduction pathof the sense VCI 2020 is connected differently in FIG. 6 than in FIG. 4. In particular, the conduction channel of the select VCI 2038 connectsthe conduction channel of the sense VCI 2020 to the reference signalsupply 2028 which provides a supply voltage V_(ref). Thus, theconduction channel of the sense VCI 2020 is connected in series betweenthe conduction channel of the select VCI 2038 and the input of theread-out circuit for the column. The select VCI 2038 therefore acts as aswitch that, when open, connects the sense VCI 2020 between, V_(ref),the reference signal supply 2028 and the input of the read-out circuitand, when closed, disconnects the sense VCI from the reference signalsupply 2028. In the interests of clarity, the connection between theconduction channel of the select VCI and V_(ref), the output of thereference signal supply 2028 is shown only in the top row of the arrayof pixels. The connection reference signal supply 2028 in the lower rowsof the array is indicated in the drawing using the label V_(ref).

The select VCI 2038 is therefore operable to inhibit the provision ofsignal from any inactive pixel to the input of the read-out circuit2026. This can help to ensure that signal is only received from activepixels (e.g. those in the row to which the gate drive pulse is beingapplied).

In an embodiment each column of pixels is virtually connected to aground or reference voltage. As such there may be no voltage differenceson each of the columns thereby minimising parasitic capacitance. Areference signal supply may apply a current-drive rather than avoltage-drive e.g. to reduce any effect parasitic capacitance could haveon the signal applied by the active pixels on the inputs of the read-outcircuit 2026. Alternatively, a voltage-drive may be provided and a senseVCI (optionally in combination with a select VCI) may in turn beoperated to provide a current output from the pixel.

The gate drive channel for the pixel row is connected to the first plateof the reference capacitor 2016, and to the control terminal of a selectVCI 2038. As in the pixel illustrated in FIGS. 4 and 5 , the connectionto the reference capacitor 2016 and capacitor sensing electrode 2014means that the gate drive voltage is divided between the referencecapacitor 2016 and the capacitive sensing electrode 2014 to provide theindicator voltage which controls the sense VCI 2020. The connection tothe control terminal 2040 of the select VCI 2038 however means that,when the pixel is not active, the conduction path of the sense VCI 2020is disconnected from the reference signal supply 2028.

A control terminal 2022 of the sense VCI 2020 is connected to the secondplate of the reference capacitor 2016. The conduction path of the senseVCI 2020 connects the reference signal supply 2028 to the input of theread-out circuit 2026 for the pixel's column.

A conduction path of the reset VCI 2034 is connected between the secondplate of the reference capacitor 2016 and a voltage output of the resetvoltage provider, for receiving the reset voltage. The control terminal2032 of the reset VCI 2034 is connected to a reset signal provider, suchas the gate drive channel of another row of the sensor array. This canenable the reset VCI 2034 to discharge the reference capacitor 2016during activation of another row of the array (e.g. a row of the arraywhich is activated on the gate pulse prior to the pixel's row) or topre-charge the control terminal 2022 of the sense VCI 2020 to the resetvoltage.

Operation of the sensor array of FIG. 6 will now be described.

The gate drive circuit 2024 and the read-out circuit 2026 each receive aclock signal from the controller 2006 (and optionally one or moreadditional control signals for controlling their operation). In responseto this clock signal, the gate drive circuit 2024 activates a first gatedrive channel of the gate drive circuit 2024 to provide a gate pulse toa row of the array 2010. A control voltage is thus applied to thecontrol terminal of the select VCI 2038 of the pixels in the first row(the active row during this gate pulse).

The control voltage is also applied to the control terminal of the resetVCI 2034 of the pixels in a second row (inactive during this gatepulse).

In the first row (the active row), the conduction channel of the selectVCI 2038 is switched into a conducting state by the control voltage(e.g. that which is provided by the gate pulse). The conduction channelof the select VCI 2038 thus connects the conduction channel of the senseVCI 2020 to the reference signal supply 2028.

The control voltage is also applied to the first plate of the referencecapacitor 2016. The relative division of voltage between the sensingelectrode 2014 and the reference capacitor 2016 provides an indicatorvoltage at the connection between the reference capacitor 2016 and thecapacitive sensing electrode 2014 as described above with reference toFIGS. 4 and 5 . The indicator voltage is applied to the control terminal2022 of the sense VCI 2020 to control the impedance of the conductionchannel of the sense VCI 2020. Thus, the sense VCI 2020 connects thereference signal supply 2028 to the input channel of the read-outcircuit 2026 for that column, and presents an impedance between the twowhich indicates the capacitance of the capacitive sensing electrode2014. Please note, the reference signal supply may be provided by aconstant voltage supply. The select VCI and/or sense VCI may convert theconstant voltage reference signal supply into a current output.

A current is thus provided through the conduction path of the sense VCI2020 from the reference signal supply 2028 to the input channel of theread-out circuit 2026 for the pixel's column. This current is determinedby the voltage of the reference signal supply and by the impedance ofthe conduction channel of the sense VCI.

In response to the same clock signal from the controller 2006, theread-out circuit 2026 senses the pixel output signal at each inputchannel (e.g. by integrating the current provided to each inputchannel), and digitises this signal. The integration time of theread-out circuit 2026 may match the duration of the gate pulse.

Thus, in each clock cycle, the read-out 2026 circuit obtains a set ofdigital signals, each signal corresponding to the signals sensed fromeach column of the active row during the gate pulse. The output fromeach pixel 2012 in the row (each channel during that gate pulse) beingindicative of the charge stored on the capacitive sensing electrode inthat pixel.

In the second (inactive) row the control voltage is applied to thecontrol terminal 2032 of the reset VCI 2034. This causes the reset VCI2034 of the pixels in the inactive row to connect the second plate oftheir reference capacitors 2016 to a reset voltage provided by the resetvoltage provider. This may discharge (e.g. at least partially remove)charge accumulated on the pixels of the inactive row, or it may chargethem to the reset voltage, before they are next activated in asubsequent gate pulse. This reset voltage may be selected to tune thesensitivity of the pixels.

At the boundaries of the pixel array, where an N−1 gate line is notavailable, a dummy signal may be used to provide the control signal tothe reset VCI. The gate drive circuit 2024 may provide the dummy signal.This may be provided by a gate drive channel which is only connected tothe rest VCIs of a row at the boundary of the array, but not to anysense or select VCIs.

As illustrated in FIG. 3 , the reset VCI 2034 of the pixels may beconnected to the gate drive circuit so that each row is discharged inthis way by the gate pulse which activates the immediately precedingrow, which may be an adjacent row of the array.

It is to be appreciated in the context of the present disclosure thatreference to voltage-controlled impedances and/or to transistors are notto be considered limiting. Such components may have a conductive pathhaving a conduction dependent on a voltage at a control terminal of thatcomponent. For example, the voltage-controlled impedances may comprisetransistors such as field effect transistors (e.g. thin filmtransistors), or other components may be used. For transistors, thecontrol terminal may be a gate region, with the conductive path beingbetween drain and source. The conductive path may be selectively openedbased on the voltage at the drain region.

It will be appreciated from the discussion above that the examples shownin the figures are merely exemplary, and include features which may begeneralised, removed or replaced as described herein and as set out inthe claims. With reference to the drawings in general, it will beappreciated that schematic functional block diagrams are used toindicate functionality of systems and apparatus described herein. Inaddition, the processing functionality may also be provided by deviceswhich are supported by an electronic device. It will be appreciatedhowever that the functionality need not be divided in this way, andshould not be taken to imply any particular structure of hardware otherthan that described and claimed below. The function of one or more ofthe elements shown in the drawings may be further subdivided, and/ordistributed throughout apparatus of the disclosure. In some examples thefunction of one or more elements shown in the drawings may be integratedinto a single functional unit.

As will be appreciated by the skilled reader in the context of thepresent disclosure, each of the examples described herein may beimplemented in a variety of different ways. Any feature of any aspectsof the disclosure may be combined with any of the other aspects of thedisclosure. For example method aspects may be combined with apparatusaspects, and features described with reference to the operation ofparticular elements of apparatus may be provided in methods which do notuse those particular types of apparatus. In addition, each of thefeatures of each of the examples is intended to be separable from thefeatures which it is described in combination with, unless it isexpressly stated that some other feature is essential to its operation.Each of these separable features may of course be combined with any ofthe other features of the examples in which it is described, or with anyof the other features or combination of features of any of the otherexamples described herein. Furthermore, equivalents and modificationsnot described above may also be employed without departing from theinvention.

Certain features of the methods described herein may be implemented inhardware, and one or more functions of the apparatus may be implementedin method steps. It will also be appreciated in the context of thepresent disclosure that the methods described herein need not beperformed in the order in which they are described, nor necessarily inthe order in which they are depicted in the drawings. Accordingly,aspects of the disclosure which are described with reference to productsor apparatus are also intended to be implemented as methods and viceversa. The methods described herein may be implemented in computerprograms, or in hardware or in any combination thereof. Computerprograms include software, middleware, firmware, and any combinationthereof. Such programs may be provided as signals or network messagesand may be recorded on computer readable media such as tangible computerreadable media which may store the computer programs in non-transitoryform. Hardware includes computers, handheld devices, programmableprocessors, general purpose processors, application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), and arrays oflogic gates.

Other examples and variations of the disclosure will be apparent to theskilled addressee in the context of the present disclosure.

1. A capacitive biometric skin contact sensor configured to resolve thecontours of skin in contact with the sensor, wherein the sensorcomprises: an array of sensor pixels, wherein each sensor pixelcomprises a thin film transistor and a capacitive sensing electrodeconnected to the thin film transistor; a plurality of gate drivechannels, wherein each gate drive channel is arranged to provide a gatedrive signal to one or more of the sensor pixels; a plurality ofread-out channels, wherein each read-out channel is arranged to receivea read-out current from one or more of the sensor pixels, each read-outcurrent being indicative of a proximity to a respective capacitivesensing electrode of a conductive object to be sensed; and an analog todigital converter comprising a dual slope integrator arranged toreceive, as its input, either an output current or a reference voltage,wherein said output current is based on one or more read-out currents;wherein the biometric sensor is configured to: apply the output currentas the input to the dual slope integrator for a charging time period tocharge a capacitor of the dual slope integrator; apply the referencevoltage as the input to the dual slope integrator for a discharging timeperiod, wherein the discharging time period comprises the amount of timeit takes for the capacitor to discharge; and determine the proximity tothe one or more respective capacitive sensing electrodes of theconductive body based on an indication of the discharging time period.2. The biometric sensor of claim 1, wherein the biometric sensor isconfigured to control the charging time period.
 3. The biometric sensorof claim 2, wherein the biometric sensor is configured to increase thecharging time period to increase the sensitivity of the sensor.
 4. Thebiometric sensor of claim 1, wherein the biometric sensor comprises acurrent multiplexer connected to a plurality of the read-out channels toreceive read-out currents therefrom; and wherein the biometric sensor isconfigured to control the number of read-out currents selected by themultiplexer.
 5. The biometric sensor of claim 4, wherein the biometricsensor is configured to: control the number of read-out currentsselected by the multiplexer based on the charging time period; and/orcontrol the charging time period based on the number of read-outcurrents selected by the multiplexer.
 6. The biometric sensor of claim1, wherein the sensor comprises a current mirror assembly configured toreceive an input current indicative of the one or more read-out currentsand to provide a selected gain to the input current to provide theoutput current; and wherein the sensor is configured to control theselected gain to the input current.
 7. The biometric sensor of claim 6,wherein the biometric sensor is configured to: control the selected gainto the input current based on the charging time period; and/or controlthe charging time period based on the selected gain to the inputcurrent.
 8. The biometric sensor of claim 1, wherein the sensorcomprises: a current multiplexer connected to a plurality of theread-out channels to receive read-out currents therefrom; and a currentmirror assembly connected to the multiplexer to receive an input currenttherefrom and to provide a selected gain to the input current to providethe output current; wherein the sensor is configured to control at leastone of: (i) the number of read-out currents selected by the multiplexer,and (ii) the selected gain to the input current.
 9. The biometric sensorof claim 8, wherein at least one of: the sensor is configured to controlat least one of: (i) the number of read-out currents selected by themultiplexer, and (ii) the selected gain to the input current, based onthe charging time period; the sensor is configured to control at leastone of: (i) the number of read-out currents selected by the multiplexer,and (ii) the charging time period, based on the selected gain to theinput current; and the sensor is configured to control at least one of:(i) the selected gain to the input current, and (ii) the charging timeperiod, based on the number of read-out currents selected by themultiplexer.
 10. The biometric sensor of claim 8, wherein the sensor isconfigured to control at least one of: (i) the number of read-outcurrents selected by the multiplexer, and (ii) the selected gain to theinput current, to increase the output current when the charging timeperiod is decreased.
 11. The biometric sensor of claim 8, wherein thesensor is configured to select the resolution and/or sensitivity of thesensor by controlling at least one of: (i) the number of read-outcurrents selected by the multiplexer, (ii) the selected gain to theinput current, and (iii) the charging time period.
 12. The biometricsensor of claim 1, wherein the dual slope integrator comprises anintegrator and a comparator, wherein the capacitor is part of theintegrator; wherein the positive terminal of the comparator is connectedto a reference voltage and wherein the negative terminal of thecomparator is connected to the output from the integrator; and whereinthe output from the comparator is connected to a controller configuredto determine the discharging time period.
 13. The biometric sensor ofclaim 1, wherein each sensor pixel comprises a reference capacitor, andwherein for each sensor pixel, the reference capacitor and thecapacitive sensing electrode are connected to a gate region of the thinfilm transistor. 14-15. (canceled)
 16. A method of operating acapacitive biometric skin contact sensor configured to resolve thecontours of skin in contact with the sensor, wherein the sensorcomprises: an array of sensor pixels, wherein each sensor pixelcomprises a thin film transistor and a capacitive sensing electrodeconnected to the thin film transistor; a plurality of gate drivechannels, wherein each gate drive channel is arranged to provide a gatedrive signal to one or more of the sensor pixels; a plurality ofread-out channels, wherein each read-out channel is arranged to receivea read-out current from one or more of the sensor pixels, each read-outcurrent being indicative of a proximity to a respective capacitivesensing electrode of a conductive object to be sensed; and an analog todigital converter comprising a dual slope integrator arranged toreceive, as its input, either an output current or a reference voltage,wherein said output current is based on one or more read-out currents;wherein the method comprises: applying the output current as the inputto the dual slope integrator for a charging time period to charge acapacitor of the dual slope integrator; applying the reference voltageas the input to the dual slope integrator for a discharging time period,wherein the discharging time period comprises the amount of time ittakes for the capacitor to discharge; and determining the proximity tothe one or more respective capacitive sensing electrodes of theconductive body based on an indication of the discharging time period.17. The method of claim 16, wherein the method comprises controlling thecharging time period.
 18. The method of claim 17, wherein the methodcomprises increasing the charging time period to increase thesensitivity of the sensor.
 19. The method of claim 16, wherein thebiometric sensor comprises a current multiplexer connected to aplurality of the read-out channels to receive read-out currentstherefrom, and wherein the method comprises: controlling the number ofread-out currents selected by the multiplexer based on the charging timeperiod; and/or controlling the charging time period based on the numberof read-out currents selected by the multiplexer.
 20. The method ofclaim 16, wherein the sensor comprises a current mirror assemblyconfigured to receive an input current indicative of the one or moreread-out currents and to provide a selected gain to the input current toprovide the output current, and wherein the method comprises:controlling the selected gain to the input current based on the chargingtime period; and/or controlling the charging time period based on theselected gain to the input current.
 21. The method of claim 16, whereinthe sensor comprises: (i) a current multiplexer connected to a pluralityof the read-out channels to receive read-out currents therefrom; and(ii) a current mirror assembly connected to the multiplexer to receivean input current therefrom and to provide a selected gain to the inputcurrent to provide the output current, and wherein the method comprises:controlling the resolution, sensitivity and/or sensing time of thesensor by controlling at least one of: (i) the number of read-outcurrents selected by the multiplexer, (ii) the selected gain to theinput current, and (iii) the charging time period. 22-24. (canceled) 25.A computer program product comprising computer program instructionsconfigured to program a controller to perform the method of claim 16.